Aside from fixing MST->SST switching (KMS never turned off MST link config),
this should preserve existing behaviour for the moment, but provide a path
for the KMS driver to have more explicit control of the DP link, which has
been requested by Lyude.

More research into modeset/supervisor interactions is needed before we can
have fully explicit control from the KMS driver.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>