iommu_gas: Eliminate unnecessary wrappers

Eliminate trivial wrappers for several iommu_gas functions that serve
no functional purpose.

Reviewed by: br, dougm, kib
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35487

(cherry picked from commit fc2e4f15a9047bbf546cd675ed590b88e54362bd)