drm/i915: Flush the workqueue before draining

Trying to drain a workqueue while we may still be adding to it from
background tasks is, according to kernel/workqueue.c, verboten. So, add
a flush_workqueue() at the start of our cleanup procedure.

References: https://bugs.freedesktop.org/show_bug.cgi?id=110550
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703171913.16585-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -2473,6 +2473,7 @@ static inline void i915_gem_drain_workqueue(struct drm_i915_private *i915)
 	 */
 	int pass = 3;
 	do {
+		flush_workqueue(i915->wq);
 		rcu_barrier();
 		i915_gem_drain_freed_objects(i915);
 	} while (--pass);