drm/xe/pf: Disable PF restart worker on device removal

We can't let restart worker run once device is removed, since other
data that it might want to access could be already released.
Explicitly disable worker as part of device cleanup action.

Fixes: a4d1c5d0 ("drm/xe/pf: Move VFs reprovisioning to worker")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://lore.kernel.org/r/20250801142822.180530-2-michal.wajdeczko@intel.com