drm/i915: Generalise wait for execlists to be idle

The code to check for execlists completion is generic, so move it to
intel_engine_cs.c, where we can reuse the new intel_engine_is_idle().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303121947.20482-2-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>