drm/mediatek: Rename "mtk_drm_gem" to "mtk_gem"

drm/mediatek: Rename "mtk_drm_gem" to "mtk_gem"

Rename all "mtk_drm_gem" to "mtk_gem":
- To align the naming rule
- To reduce the code size

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/20240322091232.26387-5-shawn.sung@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>