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