powerpc/dma: remove get_pci_dma_ops

This function is only used by the Cell iommu code, which can keep track
if it is using the iommu internally just as good.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>