[AGPGART] fix detection of aperture size versus GTT size on G965

On the G965, the GTT size may be larger than is required to cover the
aperture.  (In fact, on all hardware we've seen, the GTT is 512KB to the
aperture's 256MB).  A previous commit forced the aperture size to 512MB on
G965 to match GTT, which would likely result in hangs at best if users
tried to rely on agpgart's aperture size information.  Instead, we use the
resource length for the aperture size and the system's reported GTT size
when available for the GTT size.

Because the MSAC registers which had been read for aperture size detection
on i9xx chips just cause a change in the resource size, we can use generic
code for aperture detection on all i9xx.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Jones <davej@redhat.com>