msm: vidc: Avoid memset of list elements in the buffer entry.

- Due to race condition between free buffer and flush output buffers,
  invalid buffer is getting accessed sometimes.
- Reset all the members in buffer entry except list elements.

Change-Id: Ia2a3bc4343fc557a6e6d7d9ad93a29a7ab3dfb81
CRs-Fixed: 411197
Signed-off-by: Srinu Gorle <sgorle@codeaurora.org>