client: wake-up threads on all return paths from read_events

If wl_connection_read returned EAGAIN, we must wake up sleeping
threads. If we don't do this and the thread calling
wl_connection_read won't call wl_display_read_events again,
the sleeping threads will sleep indefinitely.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>