drm/xe: Drop bo->size

bo->size is redundant because the base GEM object already has a size
field with the same value. Drop bo->size and use the base GEM object’s
size instead. While at it, introduce xe_bo_size() to abstract the BO
size.

v2:
 - Fix typo in kernel doc (Ashutosh)
 - Fix kunit (CI)
 - Fix line wrap (Checkpatch)
v3:
 - Fix sriov build (CI)
v4:
 - Fix display build (CI)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://lore.kernel.org/r/20250625144128.2827577-1-matthew.brost@intel.com