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>