drm/i915: Extract ilk_csc_limited_range()

Extract a helper which determines if we need to use the
pipe CSC for limited range RGB output.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190218193137.22914-4-ville.syrjala@linux.intel.com


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