drm/nouveau/kms/nv50-: Use atomic encoder callbacks everywhere

It turns out that I forgot to go through and make sure that I converted all
encoder callbacks to use atomic_enable/atomic_disable(), so let's go and
actually do that.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Fixes: 09838c4e ("drm/nouveau/kms: Search for encoders' connectors properly")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>