drm/i915: Extract ilk_csc_convert_ctm()

Start splitting low level nuts and bolts stuff from
ilk_load_csc_matrix(). The goal is to leave only the clear
high level logic in place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190218193137.22914-6-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>