drm/mediatek: disable all the planes in atomic_disable

Under shadow register case, we do not disable all the plane before
disable all the hardwares. Fix it.

Fixes: 9dc84e98 ("drm/mediatek: add shadow register support")

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>