powerpc/dma: remove get_dma_offset

Just fold the calculation into __phys_to_dma/__dma_to_phys as those are
the only places that should know about it.

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>