x86/amd_gart: remove the mapping_error dma_map_ops method

Return DMA_MAPPING_ERROR instead of the magic bad_dma_addr on a dma
mapping failure and let the core dma-mapping code handle the rest.

Remove the magic EMERGENCY_PAGES that the bad_dma_addr gets redirected to.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>