x86/dma/amd_gart: Look at dev->coherent_dma_mask instead of GFP_DMA

We want to phase out looking at the magic GFP_DMA flag in the DMA mapping
routines, so switch the gart driver to use the dev->coherent_dma_mask
instead, which is used to select the GFP_DMA flag in the caller.

Tested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Jon Mason <jdmason@kudzu.us>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Muli Ben-Yehuda <mulix@mulix.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: iommu@lists.linux-foundation.org
Link: http://lkml.kernel.org/r/20180319103826.12853-6-hch@lst.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>