drm/i915: implement sync_audio_rate callback

HDMI audio may not work at some frequencies
with the HW provided N/CTS.

This patch sets the proper N value for the
given audio sample rate at the impacted frequencies.
At other frequencies, it will use the N/CTS value
which HW provides.

Signed-off-by: Libin Yang <libin.yang@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index e1db8de..22dd704 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1885,6 +1885,11 @@
 	/* hda/i915 audio component */
 	struct i915_audio_component *audio_component;
 	bool audio_component_registered;
+	/**
+	 * av_mutex - mutex for audio/video sync
+	 *
+	 */
+	struct mutex av_mutex;
 
 	uint32_t hw_context_size;
 	struct list_head context_list;