drm/i915: NULL aliasing_ppgtt on cleanup

This will allow us to carry on if we've cleaned up the PPGTT. The usage
for this is coming up - it simplifies handling a failed PPGTT init.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Spill the secrets about failing ppgtt init.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -316,6 +316,7 @@ void i915_gem_cleanup_aliasing_ppgtt(struct drm_device *dev)
 		return;
 
 	ppgtt->cleanup(ppgtt);
+	dev_priv->mm.aliasing_ppgtt = NULL;
 }
 
 void i915_ppgtt_bind_object(struct i915_hw_ppgtt *ppgtt,