drm/nouveau/kms: Move drm_dp_cec_unset_edid() into nouveau_connector_detect()

For whatever reason we currently unset the EDID for DP CEC support when
responding to the connector being unplugged, instead of just doing it in
nouveau_connector_detect() where we set the CEC EDID. This isn't really
needed and could even potentially cause us to forget to unset the EDID
if the connector is removed without a corresponding hpd event, so let's
fix that.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-12-lyude@redhat.com