intel-gtt: introduce pte write function for i8xx/i915/i945

And put it to use in the gtt configuration code that writes
the scratch page addr in all gtt ptes. This makes intel_i830_configure
generic, hence rename it to intel_fake_agp_configure.

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