drm/i915/chv: Skip gen6_gt_check_fifodbg() on CHV

CHV uses the gen8 shadow register mechanism so we shouldn't be
checking the GT FIFO status.

This effectively removes the posting read, so add an explicit
posting read using FORCEWAKE_ACK_VLV (which is what use in
vlv_forcewake_reset()).

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>