drm/i915/gtt: Don't restore the non-existent PDE for GGTT

On resume, we have to rewrite all the PDE entries for gen7 ppgtts. If we
switch on full-ppgtt, there is then one address space with no PDE, the
GGTT. Currently under aliasing-ppgtt, the GGTT address space does have
an associated ppgtt and so the restore works just fine. We would have a
similar problem if we tried disabling aliasing-ppgtt
(i915.enable_ppgtt=0). So skip the empty ppgtt, as being non-existent it
doesn't need restoring.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180601093554.13083-2-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3665,6 +3665,8 @@ void i915_gem_restore_gtt_mappings(struct drm_i915_private *dev_priv)
 				ppgtt = dev_priv->mm.aliasing_ppgtt;
 			else
 				ppgtt = i915_vm_to_ppgtt(vm);
+			if (!ppgtt)
+				continue;
 
 			gen6_write_page_range(ppgtt, 0, ppgtt->base.total);
 		}