drm/i915/audio: Protect singleton register with a lock

On the "ilk" platforms AUD_CNTL_ST2 is a singleton. Protect
it with the audio mutex in case we ever want to do parallel
RMW access to it.

Currently that should not happen since we only do audio
enable/disable from full modesets, and those are fully
serialized. But we probably want to think about toggling
audio on/off from fastsets too.

The hsw codepaths already have the same locking.
g4x should not need it since it can only do audio to a
single port at a time, which means it's actually broken
in more ways than this atm.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026170150.2654-8-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_audio.c

@@ -707,6 +707,8 @@ static void ilk_audio_codec_disable(struct intel_encoder *encoder,
 
 	ilk_audio_regs_init(i915, pipe, &regs);
 
+	mutex_lock(&i915->display.audio.mutex);
+
 	/* Disable timestamps */
 	tmp = intel_de_read(i915, regs.aud_config);
 	tmp &= ~AUD_CONFIG_N_VALUE_INDEX;
@@ -721,6 +723,8 @@ static void ilk_audio_codec_disable(struct intel_encoder *encoder,
 	tmp = intel_de_read(i915, regs.aud_cntrl_st2);
 	tmp &= ~IBX_ELD_VALID(port);
 	intel_de_write(i915, regs.aud_cntrl_st2, tmp);
+
+	mutex_unlock(&i915->display.audio.mutex);
 }
 
 static void ilk_audio_codec_enable(struct intel_encoder *encoder,
@@ -749,6 +753,7 @@ static void ilk_audio_codec_enable(struct intel_encoder *encoder,
 
 	ilk_audio_regs_init(i915, pipe, &regs);
 
+	mutex_lock(&i915->display.audio.mutex);
 
 	/* Invalidate ELD */
 	tmp = intel_de_read(i915, regs.aud_cntrl_st2);
@@ -781,6 +786,8 @@ static void ilk_audio_codec_enable(struct intel_encoder *encoder,
 	else
 		tmp |= audio_config_hdmi_pixel_clock(crtc_state);
 	intel_de_write(i915, regs.aud_config, tmp);
+
+	mutex_unlock(&i915->display.audio.mutex);
 }
 
 /**