iommu/amd: Split IOMMU group allocation and attach

Add a WARN_ON to make it clear why we don't add dma_pdev->dev to the
group we're allocating.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>