xcb-client-helper: Call xcb_wait_for_event directly

We're currently calling ppoll() before calling xcb_wait_for_event(), which
may be due to initially trying to make this non-blocking.

However, xcb_wait_for_event() reads all events available - even if there
are more than one.

There are a handful of X properties we're sent that we don't explicitly
ask for, and if these end up in the same read, we could theoretically
end up in a poll() with nothing coming in.

Drop the extra ppoll() and just let xcb_wait_for_event() do the blocking
for us.

I'm hoping this fixes the occasional timeout in the xwayland test, but
it's a reasonable code simplification even if it doesn't.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>