cleanup the dma_pgprot handling

Hi all,

this series replaced the arch_dma_mmap_pgprot hooks with the
simpler pgprot_dmacoherent as used by the arm code already and
cleans up various bits around that area.

I'd still like to hear a confirmation from the mips folks how
the write combibe attribute can or can't work with the KSEG1
uncached segment.