AMD-vi: Mixed format IVHD block should replace fixed format IVHD block

This fixes double IVHD_SETUP_INTR calls on the same IOMMU device.

Sponsored by: The FreeBSD Foundation
MFC with: 74ada297e897
Reported by: Oleg Ginzburg <olevole@olevole.ru>
Reviewed by: grehan
Approved by: philip (mentor)
Differential Revision: https://reviews.freebsd.org/D29521

(cherry picked from commit 03efa462b2ab3ae8166598363e9e83d4e5cf0398)