x86: move pci-nommu's dma_mask check to common code

The check to see if dev->dma_mask is NULL in pci-nommu is more
appropriate for dma_alloc_coherent().

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/asm-x86/dma-mapping.h b/include/asm-x86/dma-mapping.h
index 3a9a6f5..088c568 100644
--- a/include/asm-x86/dma-mapping.h
+++ b/include/asm-x86/dma-mapping.h
@@ -254,6 +254,9 @@
 		gfp |= GFP_DMA;
 	}
 
+	if (!dev->dma_mask)
+		return NULL;
+
 	if (ops->alloc_coherent)
 		return ops->alloc_coherent(dev, size,
 				dma_handle, gfp);