drm/i915: enable rc6 by default

With FBC disabled by default, it should be safe to enable RC6.  So let's
give it a try.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>