drm/i915: enable PPGTT on VLV

Working for real this time.  i915_ppgtt_info has all sorts of good stuff
in it and X is running nicely on top.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>