fix default dma_mmap_* pgprot

[Christoph Hellwig]

Hi all,

As Shawn pointed out we've had issues with the dma mmap pgprots ever
since the dma_common_mmap helper was added beyong the initial
architectures - we default to uncached mappings, but for devices that
are DMA coherent, or if the DMA_ATTR_NON_CONSISTENT is set (and
supported) this can lead to aliasing of cache attributes.  This patch
fixes that.  My explanation of why this hasn't been much of an issue
is that the dma_mmap_ helpers aren't used widely and mostly just in
architecture specific drivers.