mediatek-drm maintainers and gamma correction

- add MAINTAINERS entry for mediatek-drm driver
- add support for AAL and GAMMA engines
- hook up gamma correction LUT
- add support for temporal dithering to OD and GAMMA engines

* tag 'mediatek-drm-next-2016-08-12' of git://git.pengutronix.de/git/pza/linux:
  drm/mediatek: set mt8173 dithering function
  drm/mediatek: Add gamma correction.
  drm/mediatek: Add GAMMA engine basic function
  drm/mediatek: Add AAL engine basic function
  drm: mediatek: add Maintainers entry for Mediatek DRM drivers