drm/xe/sa: Shadow buffer support in the sub-allocator pool

drm/xe/sa: Shadow buffer support in the sub-allocator pool

The existing sub-allocator is limited to managing a single buffer object.
This enhancement introduces shadow buffer functionality to support
scenarios requiring dual buffer management.

The changes include added shadow buffer object creation capability,
Management for both primary and shadow buffers, and appropriate locking
mechanisms for thread-safe operations.

This enables more flexible buffer allocation strategies in scenarios where
shadow buffering is required.

Signed-off-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com>
Suggested-by: Matthew Brost <matthew.brost@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251118120745.3460172-2-satyanarayana.k.v.p@intel.com