drm/i915: Avoid blocking the kworker thread on a stuck mutex

Just reschedule the retire requests again if the device is currently
busy. The request list will be pruned along other paths so will never
grow unbounded and so we can afford to miss the occasional pruning.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -1901,7 +1901,12 @@ i915_gem_retire_work_handler(struct work_struct *work)
 				mm.retire_work.work);
 	dev = dev_priv->dev;
 
-	mutex_lock(&dev->struct_mutex);
+	/* Come back later if the device is busy... */
+	if (!mutex_trylock(&dev->struct_mutex)) {
+		queue_delayed_work(dev_priv->wq, &dev_priv->mm.retire_work, HZ);
+		return;
+	}
+
 	i915_gem_retire_requests(dev);
 
 	if (!dev_priv->mm.suspended &&