drm/radeon/mst: fix regression in lane/link handling.

The function this used changed in
    092c96a8
    drm/radeon: fix dp link rate selection (v2)

However for MST we should just always train to the
max link/rate. Though we probably need to limit this
for future hw, in theory radeon won't support it.

This fixes my 30" monitor with MST enabled.

Cc: stable@vger.kernel.org # v4.4
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>