drm/xe: Add xe_vm_has_valid_gpu_mapping helper

Rather than having multiple READ_ONCE of the tile_* fields and comments
in code, use helper with kernel doc for single access point and clear
rules.

v3:
 - s/xe_vm_has_valid_gpu_pages/xe_vm_has_valid_gpu_mapping

Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://lore.kernel.org/r/20250616063024.2059829-2-matthew.brost@intel.com