x86, iommu: replace CONFIG_IOMMU with CONFIG_GART_IOMMU in iommu.h

Hmm, looks like it would be nice to have more cleanups of iommu.h and
gart.h.

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