drm/mediatek: DRM driver directly refer to sub driver's function

Some ddp component exist in both display path and other path, so
sub driver should not directly call DRM driver's function. Let
DRM driver directly refer to sub driver's function so that sub
driver need not register these function to DRM driver.

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