Set primary scanout output for dnd icon surface

It can still get unthrottled frame callbacks by not having any primary
scanout output, but presumably we want this. Alongside the code setting
it for cursor surfaces, and dragged windows.

src/state.rs

@@ -756,6 +756,10 @@ impl Common {
                     }
                 }
             }
+
+            if let Some(icon) = get_dnd_icon(seat) {
+                with_surfaces_surface_tree(&icon.surface, processor);
+            }
         }
 
         // sticky window