drm/color: document NULL values and default settings better

Brought up in a discussion for enabling gamma on fsl-dcu.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Meng Yi <meng.yi@nxp.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160926090437.22676-1-daniel.vetter@ffwll.ch