msm: vidc: Fix the filled length for flushed buffers

For the buffers which are flushed internally in the driver,
the filled length should be set to zero. This change fixes
the same.

Change-Id: I21553c7812e2c15af14097f5562dbc8e68107506
CRs-Fixed: 513940
Signed-off-by: Praneeth Paladugu <ppaladug@codeaurora.org>