vtd: Increase DRHD_MAX_UNITS

Observed on a couple Ice Lake-SP platforms (Intel Coyote Pass, Dell
R750), there are more than 8 DRHD sections enumerated in the DMAR ACPI
section. Since the previous limit was 8, this resulted in some of these
not being parsed by vtd when the iommu is initialized; in this case when
PCI devices are being passthru'd to a bhyve VM.

This omission later causes a kernel panic later in initialization when
devices could not be found in a valid DRHD scope because the DHRD
containing the device's scope was not added to vtd.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

PR: 268486
Sponsored by: Intel Corporation
Reviewed by: rew@, corvink@
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D38285