drm/mediatek: Add GAMMA engine basic function

In order to correct brightness values, we have
to support gamma funciton on MT8173. In MT8173,
we have two engines for supporting gamma function:
AAL and GAMMA. This patch add some GAMMA engine
basic function, include config, start and stop
function.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>