Revert "drm/xe/vm: drop vm->destroy_work"

This reverts commit 5b259c0d

.

Cleanup here is good, however we need to able to flush a worker during
vm destruction which might involve sleeping, so bring back the worker.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240423074721.119633-3-matthew.auld@intel.com