powerpc: Rename get_dma_direct_offset get_dma_offset

The former is no longer really accurate with the swiotlb case now
a possibility.  I also move it into dma-mapping.h - it no longer
needs to be in dma.c, and there are about to be some more accessors
that should all end up in the same place.  A comment is added to
indicate that this function is not used in configs where there is no
simple dma offset, such as the iommu case.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>