drm/radeon/kms: disable ss fixed ref divide

Seems to cause problems on certain laptops

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=24462



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>