drm/radeon/kms: need to set up ss on DP bridges as well

Makes Nutmeg DP to VGA bridges work for me.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=42490



Noticed by Jerome Glisse (after weeks of debugging).

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