powerpc/vio: Use dma ops helpers

Use the set_dma_ops helper. Instead of modifying vio_dma_mapping_ops,
just create a trivial wrapper for dma_supported.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>