powerpc/dma: remove the iommu fallback for coherent allocations

All iommu capable platforms now always use the iommu code with the
internal bypass, so there is not need for this magic anymore.

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