fix(benchmark): fix FPS with LV_DEMO_BENCHMARK_MODE_RENDER_ONLY

fixes #4030

demos/benchmark/lv_demo_benchmark.c

@@ -945,10 +945,11 @@ static void dummy_flush_cb(lv_disp_t * drv, const lv_area_t * area, lv_color_t *
         }
     }
     else if(mode == LV_DEMO_BENCHMARK_MODE_RENDER_ONLY) {
+        bool last = lv_disp_flush_is_last(drv);
+
         /*Just bypass the driver and measure the pure rendering time*/
         lv_disp_flush_ready(drv);
 
-        bool last = lv_disp_flush_is_last(drv);
         if(last) {
             uint32_t t = lv_tick_elaps(render_start_time);
             if(scene_with_opa) {