x86: move dma_*_coherent functions to include file

All the x86 DMA-API functions are defined in asm/dma-mapping.h. This patch
moves the dma_*_coherent functions also to this header file because they are
now small enough to do so.
This is done as a separate patch because it also includes some renaming and
restructuring of the dma-mapping.h file.

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