drm/i915: Tune done rc6 enabling output

Power users spot this and then get adventurous and try to adjust
module driver options. Nothing good ever came out of that, so
hide it better.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>