drm/xe/guc_submit: prevent repeated unregister

It seems that various things can trigger the lr cleanup worker,
including CAT error, engine reset and destroying the actual engine, so
seems plausible to end up triggering the worker more than once in some
cases. If that does happen we can race with an ongoing engine deregister
before it has completed, thus triggering it again and also changing the
state back into pending_disable. Checking if the engine has been marked
as destroyed looks like it should prevent this.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>