drm/xe/vma: Move pat_index to vma attributes

The PAT index determines how PTEs are encoded and can be modified by
madvise. Therefore, it is now part of the vma attributes.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250821173104.3030148-4-himal.prasad.ghimiray@intel.com


Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>