intel-gtt: consolidate i9xx setup

The only difference between i915 and i965 was the calculation of the
gtt address. So merge these two paths into one. Otherwise the same
changes as in the i830 setup consolidation.

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