x86: gart alloc_coherent doesn't need to check NULL device argument

asm/dma-mapping.h guarantees that gart alloc_coherent doesn't get NULL
device argument.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: NJoerg Roedel <joerg.roedel@amd.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/pci-gart_64.c

@@ -506,9 +506,6 @@ gart_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_addr,
 
 	align_mask = (1UL << get_order(size)) - 1;
 
-	if (!dev)
-		dev = &x86_dma_fallback_dev;
-
 	*dma_addr = dma_map_area(dev, __pa(vaddr), size, DMA_BIDIRECTIONAL,
 				 align_mask);
 	flush_gart();