drm/i915/error: use rb_entry()

To make the code clearer, use rb_entry() instead of container_of() to
deal with rbtree.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/b08fd4be9d4c45d88c158a17b854c3fd628840ed.1484816339.git.geliangtang@gmail.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1080,7 +1080,7 @@ static void error_record_engine_waiters(struct intel_engine_cs *engine,
 
 	ee->waiters = waiter;
 	for (rb = rb_first(&b->waiters); rb; rb = rb_next(rb)) {
-		struct intel_wait *w = container_of(rb, typeof(*w), node);
+		struct intel_wait *w = rb_entry(rb, typeof(*w), node);
 
 		strcpy(waiter->comm, w->tsk->comm);
 		waiter->pid = w->tsk->pid;