iommu/amd: Make sure irq remapping still works on dma init failure

Do not deinitialize the AMD IOMMU driver completly when
interrupt remapping is already in use but the initialization
of the DMA layer fails for some reason. Make sure the IOMMU
can still be used to remap interrupts.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>