dix: ensure work queues are cleared on reset

If the server resets, most client workqueues are cleaned up as the
clients are killed.

The one exception is the server's client, which is exempt from
the killing spree.

If that client has a queued work procedure active, it won't get
cleared on reset.

This commit ensures it gets cleared too.
1 job for xephyr-crash in 1 minute and 1 second (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #46103
job

00:01:01