x86: move dma_supported and dma_set_mask to pci-dma_32.c

This is the way x86_64 does, so this make them equal. They have
to be extern now in the header, and the extern definition is moved to
the common dma-mapping.h header.

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