mtk_drm_crtc_atomic_disable will send an async cmd to cmdq driver,
so it may not finish when cmdq_suspend is called sometimes.

Add wait_for_event after sending async disable plane cmd to make
sure the lastest cmd is done before cmdq_suspend.
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20211202064039.20797-2-jason-jh.lin@mediatek.com/