powerpc/dma: remove dma_nommu_get_required_mask

This function is identical to the generic dma_direct_get_required_mask,
except that the generic version also takes the bus_dma_mask account,
which could lead to incorrect results in the powerpc 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>