drm/i915: disable rc6 on haswell for now

This needs proper enablement to avoid machine hangs, so let's just avoid
it for now.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>