drm/mediatek: Clear pending flag when cmdq packet is done

In cmdq mode, packet may be flushed before it is executed, so
the pending flag should be cleared after cmdq packet is done.

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>