client: Use read preparation API in wl_display_dispatch_queue()

Instead of doing things that do the equivalent of using
wl_display_prepare_read() and friends, just use the public API. The
only semantical difference is that we will now unlock and lock the mutex
more times compared to before.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>