x86: gart iommu have direct mapping when agp is present too

move init_memory_mapping() out of init_k8_gatt.

for: http://bugzilla.kernel.org/show_bug.cgi?id=11676


    2.6.27-rc2 to rc8, apgart fails, iommu=soft works, regression

This is needed because we need to map the GART aperture even
if the GATT is not initialized.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>