drm/xe/vm: drop vm->destroy_work

Now that we no longer grab the usm.lock mutex (which might sleep) it
looks like it should be safe to directly perform xe_vm_free when vm
refcount reaches zero, instead of punting that off to some 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/20240412113144.259426-5-matthew.auld@intel.com