x86: kernel/pci-dma.c cleanups

This patch contains the following cleanups:
- make the following needlessly global code static:
  - dma_alloc_pages()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>