iommu/amd: Initialize device table after dma_ops

When dma_ops are initialized the unity mappings are
created. The init_device_table_dma() function makes sure DMA
from all devices is blocked by default. This opens a short
window in time where DMA to unity mapped regions is blocked
by the IOMMU. Make sure this does not happen by initializing
the device table after dma_ops.

Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <joro@8bytes.org>