sandbox: video: Speed up video output

At present there are many situations where sandbox syncs the display to
the SDL frame buffer. This is a very expensive operation but is only
needed every now and then. Update video_sync() so that we can specify
whether this operation is really needed.

At present this flag is not used on other architectures. It could also
be used for reducing writeback-cache flushes but the benefit of that would
need to be investigated.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
diff --git a/include/video.h b/include/video.h
index e7fc5c9..cd5558f 100644
--- a/include/video.h
+++ b/include/video.h
@@ -131,8 +131,10 @@
  * buffer are displayed to the user.
  *
  * @dev:	Device to sync
+ * @force:	True to force a sync even if there was one recently (this is
+ *		very expensive on sandbox)
  */
-void video_sync(struct udevice *vid);
+void video_sync(struct udevice *vid, bool force);
 
 /**
  * video_sync_all() - Sync all devices' frame buffers with there hardware