drm/i915/vlv: disable PPGTT on early revs v3

Early revs didn't have PPGTT support, so disable there.

v2: add debug msg when disabling on early stepping
v3: enable on other B3 packages as well (untested) (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=79669
References: https://bugs.freedesktop.org/show_bug.cgi?id=79670


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