drm/vc4: Only add gamma properties once.

Two calls were made to drm_crtc_enable_color_mgmt to add gamma
and CTM, however they were both set to add the gamma properties,
so they ended up added twice.

Fixes: 766cc6b1 "drm/vc4: Add CTM support"
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>