drm/nouveau/kms: Search for encoders' connectors properly

While the way we find the associated connector for an encoder is just
fine for legacy modesetting, it's not correct for nv50+ since that uses
atomic modesetting. For reference, see the drm_encoder kdocs.

Fix this by removing nouveau_encoder_connector_get(), and replacing it
with nv04_encoder_get_connector(), nv50_outp_get_old_connector(), and
nv50_outp_get_new_connector().

v2:
* Don't line-wrap for_each_(old|new)_connector_in_state in
  nv50_outp_get_(old|new)_connector() - sravn
v3:
* Fix potential uninitialized usage of nv_connector (needs to be
  initialized to NULL at the start). Thanks kernel test robot!
v4:
* Actually fix uninitialized nv_connector usage in
  nv50_audio_component_get_eld(). The previous fix wouldn't have worked
  since we would have started out with nv_connector == NULL, but
  wouldn't clear it after a single drm_for_each_encoder() iteration.
  Thanks again Kernel bot!

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-7-lyude@redhat.com