amd-iommu: detach device explicitly before attaching it to a new domain

This fixes a bug with a device that could not be assigned to a KVM guest
because it is still assigned to a dma_ops protection domain.

[chrisw: simply remove WARN_ON(), will always fire since dev->driver
will be pci-sub]

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>