[POWERPC] Split out the IOMMU logic from cell_dma_dev_setup()

Split the IOMMU logic out from cell_dma_dev_setup() into a separate
function.  If we're not using dma_direct_ops or dma_iommu_ops we don't
know what the hell's going on, so BUG.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>