intel-gtt: introduce intel_gtt_driver

Same idea as INTEL_INFO from drm/i915. This
- reduces the dependancy on agp_driver
- stops the what-does-IS_I965G-mean confusion (here it's just gen4, in
  drm/i915 it's gen >=4)
- further prepares the separation of the fake agp driver from the rest.

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