radeon/kms: fix powerpc/rn50 untiled behaviour.

Installing 2.6.34 on a Power5/rn50 combo machine, X showed buggy sw rendering,
enabling tiling in the DDX fixed it. Investigation showed that a further /16
was needed in the untiled case on this chipset. Need further investigations
on what other chips this could affect, possibly rv100->rv280.

Signed-off-by: Dave Airlie <airlied@redhat.com>