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_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index ab37d11..990f656 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -832,6 +832,7 @@
 	mutex_init(&dev_priv->sb_lock);
 	mutex_init(&dev_priv->modeset_restore_lock);
 	mutex_init(&dev_priv->csr_lock);
+	mutex_init(&dev_priv->av_mutex);
 
 	intel_pm_setup(dev);