Revert "x86: Enable Intel DMAR by default"

A number of people have reported panics with it enabled by default,
possibly due to broken ACPI tables, which we do not handle well. D46382
is a potential fix for this issue.

Additionally DMAR is currently not compatible with bhyve passthrough
(see comment #10 in PR280817), with a draft patch to address that in
D25672.

Revert to disabling DMAR by default pending the resolution of those two
issues.

This reverts commit 3192fc30230ae432b80cca783abc2dbea9d3f383.

PR: 280817
Sponsored by: The FreeBSD Foundation