drm: Don't stack sprite planes above primary on init

We don't need to do this, we can just leave them in the plane list until
they're used.

Also, doing so helps for when we want to move the primary_plane from
the compositor to the outputs in the future.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/backend-drm/drm.c

@@ -1206,7 +1206,7 @@ create_sprites(struct drm_device *device)
 		if (drm_plane->type == WDRM_PLANE_TYPE_OVERLAY)
 			weston_compositor_stack_plane(b->compositor,
 						      &drm_plane->base,
-						      &b->compositor->primary_plane);
+						      NULL);
 	}
 
 	wl_list_for_each (drm_plane, &device->plane_list, link)