intel-gtt: consolidate i830 setup

Slighlty reordered sequence was necessary. Also don't set
agp_bridge->gatt_bus_addr anymore. Only used by generic agp helper
functions, hence unnecessary for the intel fake agp driver.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>