drm/mediatek: Add cmdq_handle in mtk_crtc

One mtk_crtc need just one cmdq_handle, so add one cmdq_handle
in mtk_crtc to prevent frequently allocation and free of
cmdq_handle.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>