drm/mediatek: Rename file mtk_drm_ddp to mtk_mutex

After mmsys routing function is moved out of mtk_drm_ddp.c, mtk_drm_ddp.c
has only mtk mutex function, so rename it to match the function in it.

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