drm/nouveau/kms/nv50-: Use nouveau_encoder->crtc in get_eld callback

drm_encoder->crtc is deprecated for atomic drivers, but
nouveau_encoder->crtc is safe.

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