drm/nouveau/kms/nv50-: Remove nv50_mstc_best_encoder()

When drm_connector_helper_funcs->atomic_best_encoder is defined,
->best_encoder is ignored by the atomic modesetting helpers. That being
said, this hook is completely broken anyway - it always returns the
first msto for a given mstc, despite the fact it might already be in
use.

So, just get rid of it. We'll need this in a moment anyway, when we make
mstos per-head as opposed to per-connector.

Changes since v1:
* Fix typo in documentation - imirkin

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>