dma-coherent: export dma_[alloc|release]_from_coherent methods

fixes modular builds:

  ERROR: "dma_alloc_from_coherent" [sound/core/snd-page-alloc.ko] undefined!
  ERROR: "dma_release_from_coherent" [sound/core/snd-page-alloc.ko] undefined!

Signed-off-by: Ingo Molnar <mingo@elte.hu>