drm/i915: enable dp mst audio

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.

This patch is ported from
commit 3708d5e0 ("drm/i915: start adding dp mst audio")

And because commit 3708d5e0 ("drm/i915: start adding dp mst audio")
breaks MST multi-monitor setups on some platforms, the orignal patch is
reverted by
commit be754b10 ("Revert "drm/i915: start adding dp mst audio"")

As the multi-monitor setups issue is fixed, let's port the patch and
enable the dp mst audio.

Signed-off-by: Libin Yang <libin.yang@intel.com>
Cc: Lyude <cpaul@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480334827-112273-3-git-send-email-libin.yang@intel.com