drm/i915: Skip gunit save/restore for cherryview

With cherryview onwards, Gunit hardware itself save and restore all the
Gunit registers. Skipping the "vlv_save_gunit_s0ix_state" &
"vlv_restore_gunit_s0ix_state" for cherryview in S3/S0ix sequence.

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>