drm/i915: Don't mangle the CTM on pre-HSW

drm/i915: Don't mangle the CTM on pre-HSW

On pre-HSW we have dedicated hardware for the RGB limited range
handling, and so we don't want to compress with the CSC matrix.

Toss in a FIXME about gamma LUT vs. limited range using the CSC.

Cc: Johnson Lin <johnson.lin@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222214232.6064-4-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>