iommu/amd: Fix wrong assumption in iommu-group specific code

The new IOMMU groups code in the AMD IOMMU driver makes the
assumption that there is a pci_dev struct available for all
device-ids listed in the IVRS ACPI table. Unfortunatly this
assumption is not true and so this code causes a NULL
pointer dereference at boot on some systems.

Fix it by making sure the given pointer is never NULL when
passed to the group specific code. The real fix is larger
and will be queued for v3.7.

Reported-by: Florian Dazinger <florian@dazinger.net>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>