drm/i915: Fix missing return warning for !CONFIG_DEBUGFS

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>