drm/vc4: Add support for gamma ramps.

We could possibly save a bit of power by not requesting gamma
conversion when the ramp happens to be 1:1, but at least if all the
CRTCs are off the SRAM will be disabled.

This should fix brightness sliders in a lot of fullscreen games.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit e582b6c7)