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

Bug: 36130225
Change-Id: If5e89703b2dec0aee8acb7e897e9df94227af3f3
Author: Praveen Chavan<pchavan@codeaurora.org>