drm/xe: Use __counted_by for flexible arrays

Good practice to use __counted_by in kernel coding for flexible arrays.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241018030039.1077842-1-matthew.brost@intel.com