drm/i915: Use DPINVGTT_STATUS_MASK

Some has given a name for the DPINVGTT status bitmask, so let's use it
instead of the magic number. Looks more like the chv code now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>