drm/xe/sa: Allow creating suballocator with custom guard size

Actual xe_sa_manager implementation uses hardcoded 4K to exclude
it from making suballocations but in upcoming patch we want to
reuse the xe_sa_manager where such 4K guard is not needed. Add
another variant of the xe_sa_bo_manager_init() function that
accepts arbitrary guard size.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241220194205.995-7-michal.wajdeczko@intel.com