drm/mediatek: Rename files "mtk_drm_plane.c" to "mtk_plane.c"

Rename files mtk_drm_plane.c to mtk_plane.c and
modify the Makefile accordingly.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.corp-partner.google.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240322091232.26387-12-shawn.sung@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: Hsiao Chien Sung

drivers/gpu/drm/mediatek/Makefile

@@ -12,12 +12,12 @@ mediatek-drm-y := mtk_crtc.o \
 		  mtk_disp_rdma.o \
 		  mtk_drm_drv.o \
 		  mtk_drm_gem.o \
-		  mtk_drm_plane.o \
 		  mtk_dsi.o \
 		  mtk_dpi.o \
 		  mtk_ethdr.o \
 		  mtk_mdp_rdma.o \
-		  mtk_padding.o
+		  mtk_padding.o \
+		  mtk_plane.o
 
 obj-$(CONFIG_DRM_MEDIATEK) += mediatek-drm.o