BIOS should be setting this, but in case it doesn't...

v2: Define the bits we actually want to clear (Jesse)
Make it an RMW op (Jesse)
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>