surfaceflinger: Exclude video encoder usage for scratch buffer

Sink and Scratch buffers in VDS are using same usage flags.
This causes video encoder flag to be set for scratch
buffers Also. Exclude video encoder flag from scratch
buffer usage flags as scratch buffers are used only as
write back input and not video encoder input.

Change-Id: I7763efd155a1ec4c550680c9a07e4a934b52dfe4
CRs-Fixed: 1008906