drm/xe/sa: Minor header cleanups

Drop unused struct xe_bo forward declaration and, while around,
fix unnecessary line split in xe_sa_bo_free() declaration.

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/20241220194205.995-8-michal.wajdeczko@intel.com