iommu: disable dma by default

APIC ID 255 and above require x2APIC and DMAR interrupt remapping.
FreeBSD is starting to be tested on high core count Intel systems that
meet this criteria. We're going to enable DMAR by default to support
this, so default hw.iommu.dma to 0 to avoid a significant performance
regression.

Reviewed by: kib, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42435