drm/xe: Disable garbage collector work item on SVM close

drm/xe: Disable garbage collector work item on SVM close

When an SVM is closed, the garbage collector work item must be stopped
synchronously and any future queuing must be prevented. Replace
flush_work() with disable_work_sync() to ensure both conditions are
met.

Fixes: 63f6e480d115 ("drm/xe: Add SVM garbage collector")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patch.msgid.link/20260227015225.3081787-1-matthew.brost@intel.com