msm: vidc: initialize sequence header timestamp to zero

Sequence header buffer returned had incorrect timestamp, resulting in
invalid timestamp sent to client. Since this is the first output
buffer from encoder it is initialized to zero.

Change-Id: I63b2faf81f178ffe862fd9da1e7984f64931e1b4
Signed-off-by: Ashray Kulkarni <ashrayk@codeaurora.org>