drm/xe/migrate: Update cpu page-table updates

Don't wait for GPU to be able to update page-tables using CPU. Putting
ourselves to sleep may be more of a problem than using GPU for
page-table updates. Also allow the vm to be NULL since the migrate
kunit test uses NULL for vm.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>