msm: vidc: Synchronize flush and release reference event

Synchronize flush command and release reference event with
instance flush lock to ensure output buffers are not queued
in release reference while waiting for flush response from
firmware.

Change-Id: Id4aa251fd52ae913f8b321918123295baf7b5b7b
Signed-off-by: Surajit Podder <spodder@codeaurora.org>