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>

test/dm/video.c

@@ -169,7 +169,7 @@ static int dm_test_video_ansi(struct unit_test_state *uts)
 
 	/* reference clear: */
 	video_clear(con->parent);
-	video_sync(con->parent);
+	video_sync(con->parent, false);
 	ut_asserteq(46, compress_frame_buffer(dev));
 
 	/* test clear escape sequence: [2J */