drm/xe: Combine destroy_cb and destroy_work in xe_vma into union

The callback kicks the worker thus mutually exclusive execution,
combining saves a bit of space in xe_vma.

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