iommu/amd: Fix missing iommu_shutdown initialization in passthrough mode

The iommu_shutdown callback is not initialized when the AMD
IOMMU driver runs in passthrough mode. Fix that by moving
the callback initialization before the check for
passthrough mode.

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