drm/i915/bdw: Ring idle and stop with logical rings

This is a hard one, since there is no direct hardware ring to
control when in Execlists.

We reuse intel_ring_idle here, but it should be fine as long
as i915_add_request does the ring thing.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>