au1200fb: fix DMA API abuse

Virtual addresses return from dma(m)_alloc_attrs are opaque in what
backs then, and drivers must not poke into them.  Similarly caching
modes are not supposed to be directly set by the driver.  Switch the
driver to use the generic DMA API mmap helper to avoid these problems.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>