spapr: Fix phb_placement backwards compatibility

spapr: Fix phb_placement backwards compatibility

When we added support for NVLink2 passthrough devices, we changed the
phb_placement hook to handle the placement of NVLink2 bridges' specific
resources.  For compatibility we use a version that doesn't do this
allocation  for old machine types.

However, because of the delay between when the patch was posted and when
it was merged, we ended up with that compatibility hook applying for
machine versions 3.1 and earlier whereas it should apply for 4.0 and
earlier (since the patch was applied early in the 4.1 tree).

Fixes: ec132efaa81 "spapr: Support NVIDIA V100 GPU with NVLink2"
Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>