x86: fix gart_iommu_init()

When the GART table is unmapped from the kernel direct mappings
during early bootup, make sure we have no leftover cachelines in it.

Note: the clflush done by set_memory_np() was not enough, because
clflush does not work on unmapped pages.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>