drm/i915: disable powersave feature for Ironlake currently

Until we figure out the right setting for powersave features on
Ironlake, disable it for now. Also disable watermark update,
which has new registers for it on Ironlake too.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[anholt: Resolved against the Pineview FBC changes]
Signed-off-by: Eric Anholt <eric@anholt.net>