[POWERPC] Remove the global dma_direct_offset

We no longer need the global dma_direct_offset, update the comment to
reflect the new reality.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>