drm/i915/chv: Add PIPESTAT register bits for Cherryview

FIXME: We probably want to sprinkle _CHV suffixes over these.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>