x86/amd-iommu: Fix crash when request_mem_region fails

When request_mem_region fails the error path tries to
disable the IOMMUs. This accesses the mmio-region which was
not allocated leading to a kernel crash. This patch fixes
the issue.

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