drm/i915: Precompute/readout/check CHV CGM mode

Let's precompute the CGM mode for CHV. And naturally we
also read it out and check it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190218193137.22914-3-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>