powerpc: Fix DMA offset for non-coherent DMA

After Becky's work we can almost have different DMA offsets
between on-chip devices and PCI. Almost because there's a
problem with the non-coherent DMA code that basically ignores
the programmed offset to use the global one for everything.
This fixes it.

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