drm/xe: Move migration support to device-level struct

drm/xe: Move migration support to device-level struct

Upcoming changes will allow users to control VF state and obtain its
migration data with a device-level granularity (not tile/gt).
Change the data structures to reflect that and move the GT-level
migration init to happen after device-level init.

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20251112132220.516975-3-michal.winiarski@intel.com
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>