drm/mediatek: apply CMDQ control flow

In some Mediatek SoC, there is no "shadow"
registers for performaing an atomic video mode
set or page flip at vblank/vsync.

The CMDQ (Commend Queue) is used to help
update all relevant display controller registers
with critical time limation.

Signed-off-by: YT Shen <yt.shen@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>