agp/intel: Restrict GTT mapping to valid range on i915 and i945

References:

  Bug 15733 - Crash when accessing nonexistent GTT entries in i915
  https://bugzilla.kernel.org/show_bug.cgi?id=15733

On G33 and above, the size of the GTT space is determined by the GMCH
control register. Prior to this revision, the size is determined by the
size of the aperture. So we must careful to map and fill the appropriate
range depending on chipset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>