Reset flushing after setting the buffer content to fix race condition.

Flushing flag is used to wait for the finish of the transfer. If this
is reset to 0, the other parts of the code will start accessing the buf_act
(see src/lv_core/lv_refr.c:106) and thus may create a race condition. Exchanging
the resetting to 0 and memset will fix this issue.

src/lv_hal/lv_hal_disp.c

@@ -267,14 +267,14 @@ bool lv_disp_get_antialiasing(lv_disp_t * disp)
  */
 LV_ATTRIBUTE_FLUSH_READY void lv_disp_flush_ready(lv_disp_drv_t * disp_drv)
 {
-    disp_drv->buffer->flushing = 0;
-
     /*If the screen is transparent initialize it when the flushing is ready*/
 #if LV_COLOR_SCREEN_TRANSP
     if(disp_drv->screen_transp) {
         memset(disp_drv->buffer->buf_act, 0x00, disp_drv->buffer->size * sizeof(lv_color32_t));
     }
 #endif
+
+    disp_drv->buffer->flushing = 0;
 }
 
 /**