drm/radeon/kms/DCE4.1: ss is not supported on the internal pplls

It's handled via external clock.  It should already be protected
by the external ss flag, but add an explicit check just in case.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>