agp/intel-gtt: move gart base addres setup

We need this thing much earlier, and it doesn't make sense
in the hw enabling function intel_enable_gtt - this does not
change over a suspend/resume cycle ...

Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>