drm/i915: Document ILK+ pipe csc matrix better

Add comments to explain the ilk pipe csc operation a bit better.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-11-ville.syrjala@linux.intel.com


Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>