drm/mediatek: Separate aal sub driver

MT8173 aal has gamma function but mt8183 aal has no gamma
function, so separate aal sub driver to have a private
data for different SoC.

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