msm: vidc: Calculate buffer timestamp and update to v4l2 buffer

By default buffer timestamp should be passed to userspace untouched.
Timestamp was being forced to zero by default, resulting in wrong
timestamp for all frames.

CRs-Fixed: 2199385
Change-Id: I2b3cdde0e0c75d119d624d232eda791fac991ca4
Signed-off-by: Chinmay Sawarkar <chinmays@codeaurora.org>