powerpc/dma: use the generic direct mapping bypass

Now that we've switched all the powerpc nommu and swiotlb methods to
use the generic dma_direct_* calls we can remove these ops vectors
entirely and rely on the common direct mapping bypass that avoids
indirect function calls entirely.  This also allows to remove a whole
lot of boilerplate code related to setting up these operations.

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