iommu/amd: Disable iommu only if amd_iommu=off is specified

It's ok to disable iommu early in normal kernel or in kdump kernel when
amd_iommu=off is specified. While we should not disable it in kdump kernel
when on-flight dma is still on-going.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>