drm/xe: Add modparam for SVM notifier size

Useful to experiment with notifier size and how it affects performance.

v3:
 - Pull missing changes including in following patch (Thomas)
v5:
 - Spell out power of 2 (Thomas)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-31-matthew.brost@intel.com