drm/omap: remove null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: David Airlie <airlied@linux.ie>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

drivers/gpu/drm/omapdrm/omap_gem.c

@@ -1179,9 +1179,7 @@ int omap_gem_op_sync(struct drm_gem_object *obj, enum omap_gem_op op)
 			}
 		}
 		spin_unlock(&sync_lock);
-
-		if (waiter)
-			kfree(waiter);
+		kfree(waiter);
 	}
 	return ret;
 }