drm/mediatek: Register vblank callback function

Some ddp component exist in both display path and other path, so
sub driver should not directly call crtc function. crtc register
callback function to sub driver to prevent sub driver directly
call crtc function.

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