What is claimed is:1. A method of manufacturing a display device, the method comprising:preparing a pixel circuit substrate including thin-film transistors and pixel electrodes respectively electrically connected to the thin-film transistors;forming a first deposition layer on the pixel circuit substrate using a first mask assembly; andforming a second deposition layer on the pixel circuit substrate using a second mask assembly,wherein:at least one of the first mask assembly and the second mask assembly includes a mask plate including an opening portion through which a deposition material passes and a covering portion that blocks a passage of the deposition material, andthe mask plate includes a protruding portion that protrudes from an edge of the covering portion toward a center region of the opening portion.2. The method as claimed in claim 1, wherein:the first mask assembly includes the mask plate and a mask sheet including a plurality of holes spaced apart from each other, andthe mask plate of the first mask assembly partially covers a first hole among the plurality of holes.3. The method as claimed in claim 2, wherein the mask plate of the first mask assembly overlaps the mask sheet and extends in a direction intersecting an extending direction of the mask sheet.4. The method as claimed in claim 1, wherein:the first deposition layer includes emission layers spaced apart from each other, anda shape of each of first emission layers among the emission layers formed on the pixel circuit substrate is different from a shape of each of second emission layers among the emission layers.5. The method as claimed in claim 4, wherein:the display device includes a transmissive area through which light or sound is transmitted, a display area that partially surrounds the transmissive area, and a non-display area between the transmissive area and the display area, andthe first emission layers correspond to the display area, and the second emission layers correspond to the non-display area.6. The method as claimed in claim 1, wherein:the first deposition layer includes emission layers spaced apart from each other, andan area of each of first emission layers among the emission layers formed on the pixel circuit substrate is greater than an area of each of second emission layers among the emission layers.7. The method as claimed in claim 1, wherein:the second mask assembly includes the mask plate,the mask plate of the second mask assembly includes an opening portion having a closed loop edge, andan edge of the protruding portion corresponds to a portion of the closed loop edge.8. The method as claimed in claim 7, wherein:the second deposition layer includes at least one of a hole transport layer, a hole injection layer, an electron transport layer, an electron injection layer, and an opposite electrode having a polarity different from that of the pixel electrodes, anda thickness of a first portion of the second deposition layer formed on the pixel circuit substrate is greater than a thickness of a second portion of the second deposition layer.9. The method as claimed in claim 8, wherein:the display device includes a transmissive area through which light or sound is transmitted, a display area that partially surrounds the transmissive area, and a non-display area between the transmissive area and the display area, andytthe first portion of the second deposition layer corresponds to the display area, and the second portion of the second deposition layer corresponds to the non-display area.10. The method as claimed in claim 9, wherein the second deposition layer is not on the transmissive area.11. A mask assembly comprising:a mask plate including:an opening portion that through which a deposition material passes,a non-closed loop covering portion that blocks a passage of the deposition material, the covering portion including a rectangular first sub-body portion and multiple second sub-body portions extending from the first sub-body portion that together form a recessed area directly connected to and facing the opening portion through which the deposition material passes, anda protruding portion in the recessed area, the protruding portion protruding from an edge of the covering portion toward a center region of the opening portion.12. The mask assembly as claimed in claim 11, wherein:the edge of the covering portion includes a linear region, andthe protruding portion protrudes from the linear region.13. The mask assembly as claimed in claim 11, wherein:the edge of the covering portion includes a curved area, andthe protruding portion protrudes from the curved area.14. The mask assembly as claimed in claim 11, wherein the covering portion and the protruding portion are integrally formed.15. A mask assembly comprising:a mask plate including:an opening portion through which a deposition material passes,a covering portion that blocks a passage of the deposition material, anda protruding portion protruding from an edge of the covering portion toward a center region of the opening portion; anda mask sheet overlapping the mask plate, the mask sheet including a plurality of holes spaced apart from each other.16. The mask assembly as claimed in claim 15, wherein the protruding portion of the mask plate partially covers the holes of the mask sheet.17. The mask assembly as claimed in claim 16, wherein the protruding portion of the mask plate partially covers a first hole among the holes of the mask sheet.18. The mask assembly as claimed in claim 15, wherein the mask sheet and the mask plate overlap each other while contacting each other.19. The mask assembly as claimed in claim 15, wherein the mask sheet and the mask plate are made of different materials.20. The mask assembly as claimed in claim 11, wherein each of the multiple second sub-body portions includes:a straight edge facing away from the first sub-body portion, and a rounded edge smoothly connected to the edge of the first sub-body portion.