drm/xe: Support device page faults on integrated platforms

Update xe_migrate_prepare_vm() to use the usm batch buffer even for
servicing device page faults on integrated platforms. And as we have
no VRAM on integrated platforms, device pagefault handler should not
attempt to migrate into VRAM.
LNL is first integrated platform to support device pagefaults.

Signed-off-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>