drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference

We are only documenting that the read is outside of the lock, and do not
require strict ordering on the operation. In this case the more relaxed
lockless_dereference() will suffice.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466581572-16608-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_fb_helper.c

@@ -464,7 +464,7 @@ static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
 
 	/* Sometimes user space wants everything disabled, so don't steal the
 	 * display if there's a master. */
-	if (READ_ONCE(dev->master))
+	if (lockless_dereference(dev->master))
 		return false;
 
 	drm_for_each_crtc(crtc, dev) {