drm/xe: Add atomic_svm_timeslice_ms debugfs entry

Add some informal control for atomic SVM fault GPU timeslice to be able
to play around with values and tweak performance.

v2:
 - Reduce timeslice default value to 5ms

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20250512135500.1405019-6-matthew.brost@intel.com