drm/i915: Extract chv_color_check()

Since CHV has the CGM unit we require a custom implementation
of .color_check().

This fixes the computation of gamma_enable as previously we
left it enabled even when were using the CGM gamma instead.
Now we turn off the legacy LUT unless it's actually required.

v2: Add some comment explaining the color pipeline (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/20190327155045.28446-5-ville.syrjala@linux.intel.com