[Intel-gfx] Prep. for DP audio MST support

[Dhinakaran Pandiyan]

This series lays the groundwork for more DP MST audio work that will
follow. Patch 1 got possibly reverted because Patch 3 was missing.

The APIs between i915 and audio drivers have to modified so that the audio
drivers can identify different displays connected to a port. The changes
introduced in Patch 2 will make that easy.

Dhinakaran Pandiyan(2)
[PATCH 2/3] drm/i915: Move audio_connector to intel_encoder
[PATCH 3/3] drm/i915: Fix enc_to_dig_port for MST encoders

Libin Yang(1)
[PATCH 1/3] drm/i915: start adding dp mst audio