drm/nouveau/kms/nv50-: Remove (nv_encoder->crtc) checks in ->disable callbacks

Noticed these in both the disable (which we'll be getting rid of in a
moment) and the atomic disable callbacks: both callback types check whether
or not there's actually a CRTC assigned to the given encoder. However, as
->atomic_disable and ->disable will never be called without a CRTC assigned
to the given encoder there's no point in this check. So just remove it.

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