x86, AMD IOMMU: initialize dma_ops after sysfs registration

If sysfs registration fails all memory used by IOMMU is freed. This
happens after dma_ops initialization and the functions will access the
freed memory then.

Fix this by initializing dma_ops after the sysfs registration.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>