drm/xe/sa: Allow making suballocations using custom gfp flags

Actual xe_sa_manager implementation uses hardcoded GFP_KERNEL flag
during creation of suballocations but in upcoming patch we want to
reuse the xe_sa_manager in places where GFP_KERNEL is not allowed.
Add another variant of the xe_sa_bo_new() function that accepts
arbitrary gfp flags.

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/20241220194205.995-6-michal.wajdeczko@intel.com