jz4740_fb: 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.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Manuel Lauss <manuel.lauss@gmail.com>
[b.zolnierkie: patch description fixup]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>