amdiommu: short-circuit all amdiommu_find_unit() functions

for the case where amdiommu is not enabled. In this case, it does not
matter if IVHD does not describe some device. Similarly, no unit can be
found for a device, because units are not instantiated, but it does not
matter.

Reported by: mav
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 3 days