audio: Fix for pop noise issue during HPX on/off

- Pop noise is heard when hpx is turned on/off
- Pop is heard as post mix commands are getting processed prior to premix
  commands in DSP due to delay of modules between premix and postmix
- Fix is to introduce fade_in after premix is on.

CRs-Fixed: 844010
Change-Id: Ibeb7ece5f8799c5119b2677e18da32df090b5ae1