sdm: Avoid flush for virtual display when no app layer is present.

Flicker is observed when there is no app layer present. Flush
operation is triggered, but flush does not update output buffer.
Fix is to avoid flush when there is valid output buffer (wfd scenario).

CRs-Fixed: 970754

Change-Id: I1125e376eea47bd31237a206ce0b2688ad6796d2
3 files changed