drm/i915: Restore PDEs for all VMs

In following with the old restore code, we must now restore ever PPGTT's
PDEs, since they aren't proper GEM ojbects.

v2: Rebased on BDW. Only do restore pdes for gen6 & 7

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>