data-device: Set "dnd" role on the icon surface, not the source one

The DnD source surface evidently has other role assigned, it's the
icon surface which is supposed to be a fresh one.

src/wayland/meta-wayland-data-device.c

@@ -410,7 +410,8 @@ data_device_start_drag (struct wl_client *client,
       seat->pointer.grab != &seat->pointer.default_grab)
     return;
 
-  if (meta_wayland_surface_set_role (surface,
+  if (icon_resource &&
+      meta_wayland_surface_set_role (wl_resource_get_user_data (icon_resource),
                                      META_WAYLAND_SURFACE_ROLE_DND,
                                      resource,
                                      WL_DATA_DEVICE_ERROR_ROLE) != 0)