drm/xe/migrate: Refactor xe_migrate_prepare_vm()

drm/xe/migrate: Refactor xe_migrate_prepare_vm()

Currently xe_migrate_prepare_vm() does three things.

1. Allocates pt_bo for migrate context.
2. Initializes pt_bo with actual pte details.
3. Initializes sa_manager for migrate context.

Split these implementations in their own functions for better
maintainability.

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260303101913.3576481-1-raag.jadav@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>