drm/nouveau/kms/nv50-: fix mst payload alloc fail crashing evo

Programming -1 (vc_start_slot, if alloc fails) into HW probably isn't
the best idea.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Danilo Krummrich <me@dakr.org>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919220442.202488-6-lyude@redhat.com