pci-gart needs to unmap the IOMMU aperture to prevent cache corruptions.

Switch this over to using set_memory_np() instead of clear_kernel_mapping().
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>