ASoC: Intel: Skylake: Fix not to stop sink pipe in pga pmd event

We should not stop the sink pipe in it's pmd handler for a mixin
module as this module may still be connected to other pipes.

This will be stopped and freed by current implementation on last
connected pipe unbind.

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>