drm/mediatek: Add component POSTMASK

This patch add component POSTMASK.

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