x86: gart: Add own dma_mapping_error function

GART IOMMU is the only user of bad_dma_address variable.

This patch converts GART to use the newer mechanism, fill in
->mapping_error() in struct dma_map_ops, to make
dma_mapping_error() work in IOMMU specific way.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: muli@il.ibm.com
Cc: joerg.roedel@amd.com
LKML-Reference: <1258287594-8777-2-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>