This was causing problems on a system with a large amount of RAM, where
display push buffers were being fetched incorrectly when placed in high
system memory addresses.

While this commit will resolve the issue on that particular system, the
issue will be avoided completely with another patch to more fully solve
problems with display and large amounts of system memory on Pascal.

It's still probably a good idea to disable this to prevent weird issues
in the future.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>