drm/i915: Fix file permissions

No functional change

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Fixes: f8d03ea0053b ("drm/i915: increase the tries for HDMI hotplug live status checking")
Link: http://patchwork.freedesktop.org/patch/msgid/1454023325-26265-1-git-send-email-mathew.j.martineau@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>