drm/i915/dp: Read the HPD status before trying to read the DPCD

Just like:

  Author: Damien Lespiau <damien.lespiau@intel.com>
  Date:   Wed Dec 12 19:37:22 2012 +0000

      drm/i915/hdmi: Read the HPD status before trying to read the EDID

But this time for DiplayPort.

v2: Adapt to the ibx_ name change and don't add commit hash (Chris
Wilson, Jani Nikula)
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -2248,6 +2248,8 @@ static enum drm_connector_status
 ironlake_dp_detect(struct intel_dp *intel_dp)
 {
 	struct drm_device *dev = intel_dp_to_dev(intel_dp);
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
 	enum drm_connector_status status;
 
 	/* Can't disconnect eDP, but you can close the lid... */
@@ -2258,6 +2260,9 @@ ironlake_dp_detect(struct intel_dp *intel_dp)
 		return status;
 	}
 
+	if (!ibx_digital_port_connected(dev_priv, intel_dig_port))
+		return connector_status_disconnected;
+
 	return intel_dp_detect_dpcd(intel_dp);
 }