mm-video-v4l2: venc: Avoid buffer access after free

client expects buffer to be free if free_buffer
is called, but if omx is in executing state free
buffer call will error out.
When async thread tries to copy data to client
buffer which is already freed,it leads to crash.
Added a bitmask to avoid copy to buffer after free.

Bug: 36130225
CRs-Fixed: 2106434

Author: Uma Mehta <umamehta@codeaurora.org>

Change-Id: Id439aac54ee64a65ea68b6431a9f5150255a6980