x86: dma_*_coherent rework patchset v2, fix

alloc_coherent dma_ops callback was added to GART, however, it doesn't
return a size aligned address wrt dma_alloc_coherent, as
DMA-mapping.txt defines. This patch fixes it.

This patch also removes unused gart_map_simple
(dma_mapping_ops->map_simple has gone).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>