ASoC: Intel: Skylake: Add missing PRE/POST_PMU handlers for vmixer

Some modules may be directly connected to a pipeline without a
mixer module. For these modules, we require PRE_PMU and POST_PMU
handler which will do bind between the pipelines, so add these
missing handlers.

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>