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 <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/20240322091232.26387-14-shawn.sung@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>