drm/i915: add DP support to intel_ddi_disable_port

Just a missing register. There is no problem to run this code when the
output is HDMI.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ddi.c

@@ -1138,14 +1138,23 @@ void intel_ddi_post_disable(struct intel_encoder *intel_encoder)
 	struct drm_i915_private *dev_priv = encoder->dev->dev_private;
 	enum port port = intel_ddi_get_encoder_port(intel_encoder);
 	uint32_t val;
+	bool wait = false;
 
 	val = I915_READ(DDI_BUF_CTL(port));
 	if (val & DDI_BUF_CTL_ENABLE) {
 		val &= ~DDI_BUF_CTL_ENABLE;
 		I915_WRITE(DDI_BUF_CTL(port), val);
-		intel_wait_ddi_buf_idle(dev_priv, port);
+		wait = true;
 	}
 
+	val = I915_READ(DP_TP_CTL(port));
+	val &= ~(DP_TP_CTL_ENABLE | DP_TP_CTL_LINK_TRAIN_MASK);
+	val |= DP_TP_CTL_LINK_TRAIN_PAT1;
+	I915_WRITE(DP_TP_CTL(port), val);
+
+	if (wait)
+		intel_wait_ddi_buf_idle(dev_priv, port);
+
 	I915_WRITE(PORT_CLK_SEL(port), PORT_CLK_SEL_NONE);
 }