intel-gtt: call init_gtt_init in probe function

This way create_gatt_table become dummy glue functions for the fake
agp driver - rename them accordingly (and kill the now unnecessary
i9xx copy).

With this change, the gtt initialization code is almost independant
from the agp stuff. Two things are still missing:
- the scratch page is created by the generic agp code.
- filling the whole gtt with scratch_page ptes is not yet consolidated -
  this needs abstracted pte handling, first.

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