x86: move alloc and free coherent to common header

they are the same between architectures. (except for the fact
that x86_64 has duplicate code)

move them to dma-mapping.h

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