sdm: Fix flush on virtual display

Flush is called for several reasons like
1. when there are no application layers
2. on prepare/commit failures

However for virtual display, (1) is not applicable as
we need to produce output buffer.

Change-Id: Ibe84f427d14d1ae5b6938a246c0de30660b72a37
CRs-Fixed: 902887
3 files changed