drm/i915: start adding dp mst audio

drm/i915: start adding dp mst audio

(This patch is developed by Dave Airlie <[email protected]> originally)

This patch adds support for DP MST audio in i915.

Enable audio codec when DP MST is enabled if has_audio flag is set.
Disable audio codec when DP MST is disabled if has_audio flag is set.

Another separated patches to support DP MST audio will be implemented
in audio driver.

v2:
Rebased.

Signed-off-by: Libin Yang <[email protected]>
Signed-off-by: Dhinakaran Pandiyan <[email protected]>
Reviewed-by: Lyude <[email protected]>
Signed-off-by: Rodrigo Vivi <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]