drm/i915: Extract ilk_color_check()

With everything else moved out of the way only ilk+
remains using _intel_color_check(). Streamline the logic
into ilk_color_check().

v2: Add some comments explaining we that we don't expose
    the full hardware capabilities currently (Matt)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190327155045.28446-9-ville.syrjala@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>