au1100fb: fix DMA API abuse

Virtual addresses return from dma(m)_alloc_coherent are opaque in what
backs then, and drivers must not poke into them.  Switch the driver
to use the generic DMA API mmap helper to avoid these games.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>