iommu static analysis cleanup

iommu static analysis cleanup

A static analyzer complained about a couple instances of checking a
variable against NULL after already having dereferenced it.
 - dmar_gas_alloc_region: remove the tautological NULL checks
 - dmar_release_resources / dmar_fini_fault_log: don't deref unit->regs
   unless initialized.

And while here, fix an inverted initialization check in dmar_fini_qi.

Reviewed by:	kib
Sponsored by:	Dell EMC Isilon
Differential revision:	https://reviews.freebsd.org/D20263