x86_64: Fail dma_alloc_coherent on dma less devices

This should fix an oops with PCMCIA PATA devices

	http://bugzilla.kernel.org/show_bug.cgi?id=8424

This is not a full fix for the problem, but probably
still the right thing to do.

[ I'm almost certain it's *not* the right thing to do, but it avoids an
  oops, and I want comments from others on what the right thing would
  actually be..  I suspect we should just remove the use of dma_mask
  entirely in this function, and just use coherent_dma_mask.  - Linus ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>