sgiwd93: convert to dma_alloc_noncoherent

Use the new non-coherent DMA API including proper ownership transfers.
This also means we can allocate the memory as DMA_TO_DEVICE instead
of bidirectional.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>