drm/omap: add a comment why locking is missing

unpin_worker() calls omap_framebuffer_unpin() without any locks, which
looks very suspicious. However, both pin and unpin are always called via
the driver's private workqueue, so the access is synchronized that way.

Add a comment to make this clear.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_plane.c

@@ -71,6 +71,10 @@ static void omap_plane_unpin_worker(struct drm_flip_work *work, void *val)
 			container_of(work, struct omap_plane, unpin_work);
 	struct drm_device *dev = omap_plane->base.dev;
 
+	/*
+	 * omap_framebuffer_pin/unpin are always called from priv->wq,
+	 * so there's no need for locking here.
+	 */
 	omap_framebuffer_unpin(val);
 	mutex_lock(&dev->mode_config.mutex);
 	drm_framebuffer_unreference(val);