x86: Set dma_ops to nommu_dma_ops by default

We set dma_ops to nommu_dma_ops at two different places for
x86_32 and x86_64. This unifies them by setting dma_ops to
nommu_dma_ops by default.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
LKML-Reference: <1258199198-16657-2-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>