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>