drm/i915: fix build for DEBUG_FS=n

Fix DEBUG_FS=n build broken by

commit aa7471d2
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
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -3029,10 +3029,11 @@ int i915_verify_lists(struct drm_device *dev);
 /* i915_debugfs.c */
 int i915_debugfs_init(struct drm_minor *minor);
 void i915_debugfs_cleanup(struct drm_minor *minor);
-int i915_debugfs_connector_add(struct drm_connector *connector);
 #ifdef CONFIG_DEBUG_FS
+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 void intel_display_crc_init(struct drm_device *dev) {}
 #endif