drm/i915/debugfs: HDCP capability enc NULL check

DP-MST connector encoder initializes at modeset
Adding a connector->encoder NULL check in order to
avoid any NULL pointer dereference.
intel_hdcp_enable() already handle this but debugfs
can also invoke the intel_{hdcp,hdcp2_capable}.
Handling it gracefully.

v2:
- Use necessary lock and NULL check in
  i915_hdcp_sink_capability_show. [Imre]
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210211140502.22786-1-anshuman.gupta@intel.com

drivers/gpu/drm/i915/display/intel_display_debugfs.c

@@ -2198,16 +2198,27 @@ DEFINE_SHOW_ATTRIBUTE(i915_panel);
 static int i915_hdcp_sink_capability_show(struct seq_file *m, void *data)
 {
 	struct drm_connector *connector = m->private;
+	struct drm_i915_private *i915 = to_i915(connector->dev);
 	struct intel_connector *intel_connector = to_intel_connector(connector);
+	int ret;
 
-	if (connector->status != connector_status_connected)
-		return -ENODEV;
+	ret = drm_modeset_lock_single_interruptible(&i915->drm.mode_config.connection_mutex);
+	if (ret)
+		return ret;
+
+	if (!connector->encoder || connector->status != connector_status_connected) {
+		ret = -ENODEV;
+		goto out;
+	}
 
 	seq_printf(m, "%s:%d HDCP version: ", connector->name,
 		   connector->base.id);
 	intel_hdcp_info(m, intel_connector);
 
-	return 0;
+out:
+	drm_modeset_unlock(&i915->drm.mode_config.connection_mutex);
+
+	return ret;
 }
 DEFINE_SHOW_ATTRIBUTE(i915_hdcp_sink_capability);