drm/i915: Extract check_luts()

In prepartion for per-platform color_check() functions extract the
common code into a separate function.

v2: Improve the C8 comment (Matt)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-2-ville.syrjala@linux.intel.com