powerpc/dma: use phys_to_dma instead of get_dma_offset

Use the standard portable helper instead of the powerpc specific one,
which is about to go away.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>