vmm: Tear down the IOMMU before AMD-Vi detach

Register the vmm module handler after both the bundled device drivers
and SMP. On platforms without EARLY_AP_STARTUP, SI_SUB_SMP follows
SI_SUB_DRIVERS; using the later subsystem preserves the
smp_rendezvous() requirement.

The resulting reverse unload order performs IOMMU cleanup while every
IVHD softc remains valid. Refuse an independent IVHD detach while
translation state remains initialized.

MFC after: 2 weeks