intel-gtt: s/intel_i830_init_gtt_entries/intel_gtt_stolen_entries

First simple step towards a more generic initialization. This
is needed to disentangle the agp stuff from the stuff that is
actually needed by drm/i915.

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