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>