drm/i915: Enable VLV audio chicken bit for LPE audio

The audio chicken bit (register offset 0x62f38) seems required to make
DP audio working on some machines.  At least, on Dell Wyse 3040, I
failed to get the audio unless this bit is set once.

Strangely, the bit seems necessary only once, and it persists after
that, even some power-off cycles.  The register is supposedly
write-only, so it's no evidence whether the bit keeps effect
persistently.  But, judging from the experiment, it looks enough to
set it up once at the device initialization.

The patch is basically a cut from the original patch by Pierre-Louis
Bossart.

v1->v2: drop read since it's a write-only reg.

Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

drivers/gpu/drm/i915/i915_reg.h

@@ -2062,6 +2062,9 @@ enum skl_disp_power_wells {
 #define I915_HDMI_LPE_AUDIO_SIZE	0x1000
 
 /* DisplayPort Audio w/ LPE */
+#define VLV_AUD_CHICKEN_BIT_REG		_MMIO(VLV_DISPLAY_BASE + 0x62F38)
+#define VLV_CHICKEN_BIT_DBG_ENABLE	(1 << 0)
+
 #define _VLV_AUD_PORT_EN_B_DBG		(VLV_DISPLAY_BASE + 0x62F20)
 #define _VLV_AUD_PORT_EN_C_DBG		(VLV_DISPLAY_BASE + 0x62F30)
 #define _VLV_AUD_PORT_EN_D_DBG		(VLV_DISPLAY_BASE + 0x62F34)

drivers/gpu/drm/i915/intel_lpe_audio.c

@@ -248,6 +248,11 @@ static int lpe_audio_setup(struct drm_i915_private *dev_priv)
 		goto err_free_irq;
 	}
 
+	/* enable chicken bit; at least this is required for Dell Wyse 3040
+	 * with DP outputs (but only sometimes by some reason!)
+	 */
+	I915_WRITE(VLV_AUD_CHICKEN_BIT_REG, VLV_CHICKEN_BIT_DBG_ENABLE);
+
 	return 0;
 err_free_irq:
 	irq_free_desc(dev_priv->lpe_audio.irq);