protocol: set_desync should flush

protocol: set_desync should flush

wl_subsurface.set_desync should apply the cached wl_surface state.
Otherwise, the sub-surface may be stuck: a commit on the parent surface,
if desynchronized, will not commit the sub-surface because it is
desynchronized, too. A commit on the sub-surface may not happen, if it
is waiting for the frame callback from the previous commit.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>