drm/vc4: Expose gamma as atomic property

We are an atomic driver so the gamma LUT should also be exposed as a
CRTC property through the DRM atomic color management. This will also
take care of the legacy path for us.

Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1523479755-20812-3-git-send-email-stschake@gmail.com