drm/i915: Notify GuC rc6 state

If rc6 is enabled, notify GuC so it can do proper forcewake before
command submission.

Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>