x86: clean up iommu.h and gart.h

This remove lots of duplications in iommu.h and gart.h.

The end result of this patch is:

- iommu.h is a header file for everyone related with IOMMUs.

- gart.h is the private header file. Only pci-gart_64.c and its friends
  include it.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: fujita.tomonori@lab.ntt.co.jp
Signed-off-by: Ingo Molnar <mingo@elte.hu>