drm: Make the CRTC gamma_set operation optional

Drivers for hardware without gamma support should not be forced to
implement a no-op gamma set operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>