drm/i915: Prefer to check for idleness in worker rather than sync-flush

	i915_queue_hangcheck(ring->dev);

	cancel_delayed_work_sync(&dev_priv->mm.idle_work);

	queue_delayed_work(dev_priv->wq,

			   &dev_priv->mm.retire_work,

			   round_jiffies_up_relative(HZ));

	struct drm_i915_private *dev_priv =

		container_of(work, typeof(*dev_priv), mm.idle_work.work);

	struct drm_device *dev = dev_priv->dev;

	struct intel_engine_cs *ring;

	int i;

	for_each_ring(ring, dev_priv, i)

		if (!list_empty(&ring->request_list))

			return;

	intel_mark_idle(dev);