drm/i915: Markup potential lock for i915_active

Make the lockchains more deterministic via i915_active by flagging the
potential lock.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-7-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_active.c

@@ -268,6 +268,8 @@ int i915_active_wait(struct i915_active *ref)
 	int err;
 
 	might_sleep();
+	might_lock(&ref->mutex);
+
 	if (RB_EMPTY_ROOT(&ref->tree))
 		return 0;