drm/xe: Introduce xe_tile_is_root helper

Instead of looking at the tile->id member provide a small helper
to explicitly express our intentions.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20250713103625.1964-4-michal.wajdeczko@intel.com