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

drivers/gpu/drm/i915/intel_drv.h

@@ -1175,6 +1175,8 @@ bool intel_ddi_pll_select(struct intel_crtc *crtc,
 void intel_ddi_set_pipe_settings(struct drm_crtc *crtc);
 void intel_ddi_prepare_link_retrain(struct intel_dp *intel_dp);
 bool intel_ddi_connector_get_hw_state(struct intel_connector *intel_connector);
+bool intel_ddi_is_audio_enabled(struct drm_i915_private *dev_priv,
+				 struct intel_crtc *intel_crtc);
 void intel_ddi_get_config(struct intel_encoder *encoder,
 			  struct intel_crtc_state *pipe_config);
 struct intel_encoder *