drm/i915: Flush the i915_vm_release before ggtt shutdown

As the gen6_ppgtt may refer back to the GGTT for their page-directory
slots, make sure those __i915_vm_release are completed prior to shutting
down the GGTT.

Fixes: b32fa811 ("drm/i915/gtt: Defer address space cleanup to an RCU worker")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729132412.23380-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2721,6 +2721,9 @@ static void ggtt_cleanup_hw(struct i915_ggtt *ggtt)
 
 	ggtt->vm.closed = true;
 
+	rcu_barrier(); /* flush the RCU'ed__i915_vm_release */
+	flush_workqueue(i915->wq);
+
 	mutex_lock(&i915->drm.struct_mutex);
 
 	list_for_each_entry_safe(vma, vn, &ggtt->vm.bound_list, vm_link)