sna: Only reduce gpu damage after migration

Otherwise we end up doing work for no reason, ala the regression in
grads-heat-map. However, it is important that the damage is reduced at
some point or else it may grow unbounded. Hopefully normal usage will
never hit the pathologocial case...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_accel.c

@@ -790,8 +790,8 @@ sna_pixmap_move_to_gpu(PixmapPtr pixmap)
 	__sna_damage_destroy(priv->cpu_damage);
 	priv->cpu_damage = NULL;
 
-done:
 	sna_damage_reduce(&priv->gpu_damage);
+done:
 	list_del(&priv->list);
 	return priv;
 }