drm/i915/chv: Implement WaDisableCSUnitClockGating:chv

This workaround is listed for CHV, but not for BDW. However BSpec notes
that on BDW CSunit clock gating is always disabled irrespective of the
relevant bit in the GEN6_UGCTL1 registers. For CHV however, such text
is not present in BSpec, so it seems safer to just set the bit.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>