mm-video-v4l2: venc: Protect buffer from being freed while accessing

Output buffer (in use-buffer mode) has an internal backup ion buffer.
The contents of this buffer are deep-copied in client's buffer in
the context of VideoEncCallBackThread; while this buffer can be
freed in the client thread's context.
Check the allocation bitmask before attempting to copy and
synchronize these operations by holding a lock

Fixes bug 36130225
 Security Vulnerability - Heap use after free in libOmxVenc

CRs-Fixed: 2053101

Author: Praveen Chavan<pchavan@codeaurora.org>

Change-Id: I6141e81d7dbd50bc3601c8df066fd8cbd06b4e0b