drm/i915: Fix psr sink status report.

First of all don't try to read dpcd if PSR is not even supported.

But also, if read failed return -EIO instead of reporting via a
backchannel.

v2: fix dev_priv: At this level m->private is the connector. (CI/DK)
    don't convert dpcd read errors to EIO. (DK)

Fixes: 5b7b3086 ("drm/i915/psr: Split sink status into a separate debugfs node")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180720003155.16290-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2606,13 +2606,22 @@ static int i915_psr_sink_status_show(struct seq_file *m, void *data)
 		"sink internal error",
 	};
 	struct drm_connector *connector = m->private;
+	struct drm_i915_private *dev_priv = to_i915(connector->dev);
 	struct intel_dp *intel_dp =
 		enc_to_intel_dp(&intel_attached_encoder(connector)->base);
+	int ret;
+
+	if (!CAN_PSR(dev_priv)) {
+		seq_puts(m, "PSR Unsupported\n");
+		return -ENODEV;
+	}
 
 	if (connector->status != connector_status_connected)
 		return -ENODEV;
 
-	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_STATUS, &val) == 1) {
+	ret = drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_STATUS, &val);
+
+	if (ret == 1) {
 		const char *str = "unknown";
 
 		val &= DP_PSR_SINK_STATE_MASK;
@@ -2620,7 +2629,7 @@ static int i915_psr_sink_status_show(struct seq_file *m, void *data)
 			str = sink_status[val];
 		seq_printf(m, "Sink PSR status: 0x%x [%s]\n", val, str);
 	} else {
-		DRM_ERROR("dpcd read (at %u) failed\n", DP_PSR_STATUS);
+		return ret;
 	}
 
 	return 0;