drm/xe/migrate: Do not hand-encode pte

Instead of encoding the pte, call a new vfunc from xe_vm to handle that.
The encoding may not be the same on every platform, so keeping it in one
place helps to better support them.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230927193902.2849159-5-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>