drm/xe: Combine PF and VF device data into union

There is no need to keep PF and VF data fields fully separate
since we can be only in one mode at the time. Move them into
a anonymous union to save few bytes.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20250713103625.1964-2-michal.wajdeczko@intel.com