drm/i915: Move audio_connector to intel_encoder

With DP MST, a digital_port can carry more than one audio stream. Hence,
more than one audio_connector needs to be attached to intel_digital_port in
such cases. However, each stream is associated with an unique encoder. So,
instead of creating an array of audio_connectors per port, move
audio_connector from struct intel_digital_port to struct intel_encoder.
This also simplifies access to the right audio_connector from codec
functions in intel_audio.c that receive intel_encoder.

v2: Removed locals that are not needed anymore.

v3: No code change except for minor change in context.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Lyude <cpaul@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474334681-22690-5-git-send-email-dhinakaran.pandiyan@intel.com