drm/mediatek: Change disp/ddp term to mutex in mtk mutex driver

mtk mutex is used by both drm and mdp driver, so change disp/ddp term to
mutex to show that it's a common driver for drm and mdp.

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