msm: vidc: Initialize pic_struct to unknown as default

pic_struct needs to be initialized to default,
failing to do so will set the event to insufficient
in handle_event_change().

Change-Id: Iefe48f5ae6dd3832b0328c3dec00b56ff4291182
Signed-off-by: Paras Nagda <pnagda@codeaurora.org>