drm/i915: Fix missing return warning for !CONFIG_DEBUGFS

This broken code was introduced in

commit aa7471d228eb6dfddd0d201ea9746d6a2020972a
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Apr 1 11:15:21 2015 +0300

    drm/i915: add i915 specific connector debugfs file for DPCD

v2: Drop hunk that accidentally crept in.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: François Valenduc <francoisvalenduc@gmail.com>
Reported-by: François Valenduc <francoisvalenduc@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -3117,7 +3117,8 @@ void i915_debugfs_cleanup(struct drm_minor *minor);
 int i915_debugfs_connector_add(struct drm_connector *connector);
 void intel_display_crc_init(struct drm_device *dev);
 #else
-static inline int i915_debugfs_connector_add(struct drm_connector *connector) {}
+static inline int i915_debugfs_connector_add(struct drm_connector *connector)
+{ return 0; }
 static inline void intel_display_crc_init(struct drm_device *dev) {}
 #endif