xwayland/glamor/gbm: Only use modifier gbm API if explicit

If we're using implicit modifiers, we'll pass NULL and zero modifiers.
Lets just use the legacy API directly instead.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

hw/xwayland/xwayland-glamor-gbm.c

@@ -293,6 +293,8 @@ xwl_glamor_gbm_create_pixmap(ScreenPtr screen,
             uint64_t *modifiers = NULL;
 
             xwl_glamor_get_modifiers(screen, format, &num_modifiers, &modifiers);
+
+            if (num_modifiers > 0)
                 bo = gbm_bo_create_with_modifiers(xwl_gbm->gbm, width, height,
                                                   format, modifiers, num_modifiers);
             free(modifiers);