sf: vds: Avoid HWC for certain scenarios

Reserve HWC for VDS for WFD use case. During WFD usecase, sink usage
will contain GRALLOC_USAGE_PRIVATE_WFD flag.

When HWC is avoided, all virtual displays are composed using GLES.
This means that GLES composes directly into the sink buffer and
signals the consumer. Furthermore, it is expected that any color
conversion will be handled on the consumer side.

CRs-Fixed: 908380
Change-Id: I93ff54c79ece788b6caf3f4172835d8ac1362f62