x86: integrate pci-dma.c

The code in pci-dma_{32,64}.c are now sufficiently
close to each other. We merge them in pci-dma.c.

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