drm/i915: abstract ddi being audio enabled

Prepare for using the same code for judging ddi being audio enabled.
No functional changes.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Libin Yang <libin.yang@intel.com>
Reviewed-by: Lyude <cpaul@redhat.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/1480334827-112273-2-git-send-email-libin.yang@intel.com