powerpc/dma: use the dma_direct mapping routines

Switch the streaming DMA mapping and ownership transfer methods to the
functionally identical dma_direct_ versions.  Factor the cache
maintainance helpers into the form expected by the common code for that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>