x86: remove ifdef CONFIG_GART_IOMMU in pci-dma.c

Our way to handle gart_* functions for CONFIG_GART_IOMMU and
!CONFIG_GART_IOMMU cases is inconsistent.

We have some dummy gart_* functions in !CONFIG_GART_IOMMU case and
also use ifdef CONFIG_GART_IOMMU tricks in pci-dma.c to call some
gart_* functions in only CONFIG_GART_IOMMU case.

This patch removes ifdef CONFIG_GART_IOMMU in pci-dma.c and always use
dummy gart_* functions in iommu.h.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Muli Ben-Yehuda <muli@il.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>