x86: move dma_coherent functions to pci-dma.c

They are placed in an ifdef, since they are i386 specific
the structure definition goes 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>