iommu/amd: Fix NULL pointer dereference for GPUs without upstream bridge

pci_upstream_bridge() can return NULL for devices without an upstream
bridge. The AMD GPU identity mapping quirk dereferences the returned
pointer unconditionally, causing a kernel panic during boot.

Check the pointer before accessing the bridge vendor ID.

This fixes a boot regression on a Lenovo system with an AMD PRO
A10-9700E APU. The system now boots with AMD IOMMU enabled.

Fixes: bf5bdfd1f8d4 ("iommu/amd: Force identity mode for selected GPUs only")

Signed-off-by: Jean-Christophe Guillain <jean-christophe@guillain.net>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>