x86: add alloc_coherent dma_ops callback to GART driver

[ v2 - x86: make gart_alloc_coherent return zeroed memory

  FUJITA Tomonori pointed it out that the dma_alloc_coherent function
  should return memory set to zero. This patch adds this to the GART
  implementation too. ]

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>