drm/xe/vf: Teardown VF post migration worker on driver unload

drm/xe/vf: Teardown VF post migration worker on driver unload

Be cautious and ensure the VF post-migration worker is not running
during driver unload.

v3:
 - More teardown later in driver init, use devm (Tomasz)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>
Link: https://lore.kernel.org/r/20251008214532.3442967-16-matthew.brost@intel.com