x86: isolate coherent mapping functions

i386 implements the declare coherent memory API, and x86_64 does not
it is reflected in pieces of dma_alloc_coherent and dma_free_coherent.
Those pieces are isolated in separate functions, that are declared
as empty macros in x86_64. This way we can make the code the same.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>