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.

(cherry picked from commit 8738ce85)

Fixes: xorg/xserver#670
2 jobs for server-1.20-branch in 8 minutes and 36 seconds (queued for 42 seconds)