drm/i915: Update GEN6_RP_CONTROL definitions

This matches the modern specs more accurately.

This will be used by the following patch to fix the way we display RC
status.

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