video: Call video_sync in video_clear()

There is a need to call sync when anybody asking for clearing display.
For example via cls command.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/video-uclass.c b/drivers/video/video-uclass.c
index 938e7d3..21452a1 100644
--- a/drivers/video/video-uclass.c
+++ b/drivers/video/video-uclass.c
@@ -143,7 +143,7 @@
 	if (ret)
 		return ret;
 
-	return 0;
+	return video_sync(dev, false);
 }
 
 void video_set_default_colors(struct udevice *dev, bool invert)