[POWERPC] Have cell use its own dma_direct_offset variable

Rather than using the global variable, have cell use its own variable
to store the direct DMA offset.

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>