drm/radeon/kms: add missing frac fb div flag for dce4+

The fixed ref/post dividers are set by the AdjustPll table
rather than the ss info table on dce4+.  Make sure we enable
the fractional feedback dividers when using a fixed post
or ref divider on them as well.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=29272

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>