powerpc/dma: remove dma_nommu_dma_supported

This function is largely identical to the generic version used
everywhere else.  Replace it with the generic version.

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