drm/i915: Extract bdw_color_check()

Provide a separate .color_check() for BDW+ where we currently
provide the split gamma mode etc.

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/20190327155045.28446-8-ville.syrjala@linux.intel.com