iommu/amd: Cache pdev pointer to root-bridge

At some point pci_get_bus_and_slot started to enable
interrupts. Since this function is used in the
amd_iommu_resume path it will enable interrupts on resume
which causes a warning. The fix will use a cached pointer
to the root-bridge to re-enable the IOMMU in case the BIOS
is broken.

Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>