drm/i915/chv: Add DPINVGTT registers defines for Cherryview

Due to Pipe C DPINVGTT has more bits on CHV.

v2: Fix comment to say VLV/CHV (Rafael)

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