drm/i915: Also disable PSR on Sink when disabling it on Source.

It is not a bad idea to disable the PSR feature on Sink
when we are disabling on the Source.

v2: Move dpcd write inside mutex protected area as suggested by Sonika.

Cc: Sonika Jindal <sonika.jindal@intel.com>
Suggested-by: NSonika Jindal <sonika.jindal@intel.com>
Signed-off-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: NSonika Jindal <sonika.jindal@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_psr.c

@@ -524,11 +524,15 @@ void intel_psr_disable(struct intel_dp *intel_dp)
 		return;
 	}
 
+	/* Disable PSR on Source */
 	if (HAS_DDI(dev))
 		hsw_psr_disable(intel_dp);
 	else
 		vlv_psr_disable(intel_dp);
 
+	/* Disable PSR on Sink */
+	drm_dp_dpcd_writeb(&intel_dp->aux, DP_PSR_EN_CFG, 0);
+
 	dev_priv->psr.enabled = NULL;
 	mutex_unlock(&dev_priv->psr.lock);