drm/nouveau: Fix nv50_mstc->best_encoder()

commit 7b0f61e91b6056c71649efa3204112a4b6cf5fc8 upstream.

As mentioned in the previous commit, we currently prevent new modesets
on recently-removed MST connectors by returning no encoder from our
->best_encoder() callback once the MST port has disappeared. This is
wrong however, because it prevents legacy modesetting users from being
able to disable CRTCs on MST connectors after the connector's respective
topology has disappeared.

So, fix this by instead by just always returning a valid encoder.

Changes since v2:
- Remove usage of atomic MST helper for now, since that got replaced
  with a much simpler solution

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20181008232437.5571-3-lyude@redhat.com


(cherry picked from commit e87b0bbc9f0380d403f8f2f6abba0d51c74d944f)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>