drm/mediatek: Rename files "mtk_drm_gem.c" to "mtk_gem.c"

Rename files mtk_drm_gem.c to mtk_gem.c.

Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Signed-off-by: Hsiao Chien Sung <[e-mail]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[e-mail]/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

drivers/gpu/drm/mediatek/Makefile

···
 		  mtk_disp_ovl_adaptor.o \
 		  mtk_disp_rdma.o \
 		  mtk_drm_drv.o \
-		  mtk_drm_gem.o \
 		  mtk_dsi.o \
 		  mtk_dpi.o \
 		  mtk_ethdr.o \
+		  mtk_gem.o \
 		  mtk_mdp_rdma.o \
 		  mtk_padding.o \
 		  mtk_plane.o