drm/i915: Check PSR errors instead of retrain while PSR is enabled

When a PSR error happens sink sets the PSR error register and also
set the link status to a error status.
So in the short pulse handling it was returning earlier and doing a
full detection and attempting to retrain but it fails as PSR HW is
in change of the main-link.

Just call intel_psr_short_pulse() before
intel_dp_needs_link_retrain() is not the right fix as
intel_dp_needs_link_retrain() would return true and trigger a full
detection while PSR HW is still in change of main-link.

Check for PSR active is also not safe as it could be inactive due a
frontbuffer invalidate and still doing the PSR exit sequence.

v3: added comment in intel_dp_needs_link_retrain()

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121225441.18785-2-jose.souza@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -1147,3 +1147,18 @@ void intel_psr_short_pulse(struct intel_dp *intel_dp)
 exit:
 	mutex_unlock(&psr->lock);
 }
+
+bool intel_psr_enabled(struct intel_dp *intel_dp)
+{
+	struct drm_i915_private *dev_priv = dp_to_i915(intel_dp);
+	bool ret;
+
+	if (!CAN_PSR(dev_priv) || !intel_dp_is_edp(intel_dp))
+		return false;
+
+	mutex_lock(&dev_priv->psr.lock);
+	ret = (dev_priv->psr.dp == intel_dp && dev_priv->psr.enabled);
+	mutex_unlock(&dev_priv->psr.lock);
+
+	return ret;
+}