drm/i915: Extract i9xx_color_check()

Apart from CHV the other gmch platforms don't currently
require much work in .color_check(). So let's start by
extracting i9xx_color_check().

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/20190327155045.28446-4-ville.syrjala@linux.intel.com