radeon/kms: use frac fb div on APUs

Seems to be more stable on certain monitors.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=48880

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
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