MIPS: Consolidate coherent and non-coherent dma_alloc code

Besides eliminating lots of duplication this also allows allocations with
the DMA_ATTR_NON_CONSISTENT to use the CMA allocator.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17181/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>