drm/i915: start adding dp mst audio

(This patch is developed by Dave Airlie <airlied@redhat.com> 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 <libin.yang@linux.intel.com>
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/1474334681-22690-6-git-send-email-dhinakaran.pandiyan@intel.com