agp: iommu_gfx_mapped only available if CONFIG_INTEL_IOMMU is set

Kernels with no iommu support cannot ever need the Ironlake
work-around, so never enable it in that case.

Might be better to completely remove the work-around from the kernel
in this case?

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>