drm/i915: Update the location of the ringbuffers' HWS_PGA registers for IVB.

They have been moved from the ringbuffer groups to their own group it
looks like.  Fixes GPU hangs on gnome startup.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>