x86: cleanup dma_*_coherent functions

All dma_ops implementations support the alloc_coherent and free_coherent
callbacks now. This allows a big simplification of the dma_alloc_coherent
function which is done with this patch. The dma_free_coherent functions is also
cleaned up and calls now the free_coherent callback of the dma_ops
implementation.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>