sdm: Initialize mixer, fb and display attributes for virtual display.

1. Initialize mixer, fb and display attributes appropriately for
   virtual display while setting active configuration of a display.
2. Return appropriate value from virtual display hw interface APIs
   that are not applicable.

Change-Id: Ifaba743f90a7499b5beaffc5f551b1ddfd166ab0
CRs-Fixed: 1031765
7 files changed