msm: vidc: Do not hold FTB in driver while flush is in progress

There is no real need for the video driver to hold an output
buffer while flush is in progress. This change fixes an issue
where driver ends up queuing a held output buffer to firmware,
just after flush done. So firmware never returns the output
buffer and client ends up waiting indefinitely for the buffer.

Change-Id: I20f76b64535ca8665fa214746582f13a0acc3c5d
Signed-off-by: Arun Menon <avmenon@codeaurora.org>