drm/i915: disable RC6 when disabling rps

We weren't disabling RC6 bits when bringing down RPS.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>