drm/xe: Standardize pat_index to u16 type

drm/xe: Standardize pat_index to u16 type

Ensure all pat_index definitions consistently use u16 type across
the XE driver. This addresses two remaining instances where pat_index
was incorrectly typed:

- xe_vm_snapshot structure used int for pat_index field
- xe_device pat.idx array used u32 instead of u16

This cleanup improves type consistency and ensures proper alignment
with the PAT subsystem design.

Signed-off-by: Xin Wang <x.wang@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260416045526.536497-2-x.wang@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>