drm/mediatek: Move cmdq_reg info from struct mtk_ddp_comp to sub driver private data

Some ddp component exist in both display path and other path, so data
belonged to sub driver should be moved into sub driver private data so it
could be used for multiple path. cmdq_reg info is one of sub driver data,
so move it.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>