dmar: Disable protected memory regions after initialization

Some BIOSes protect memory region they reside in by using DMAR to
prevent devices from doing any DMA transactions to that part of RAM.
AMI refers to this as "DMA Control Guarantee".
Disable the protection when address translation is enabled.
I stumbled upon this while investigation a failing coredump on a device
which has this feature enabled.

Sponsored by: Stormshield
Obtained from: Semihalf
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D32591