drm/i915: add DOC: headline to RC6 kernel-doc

Without the DOC:, kernel-doc confuses the documentation block for
something else.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453101588-18008-3-git-send-email-jani.nikula@intel.com