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
Test: build & boot
Test: cts-tradefed run cts-dev --module CtsMediaTestCases --compatibility:module-arg
CtsMediaTestCases:include-annotation:android.platform.test.annotations.RequiresDevice

Change-Id: I4eb44837f9b3f8f1b8b2b4c879d8c3dd470bb52f
Author:    Uma Mehta <umamehta@codeaurora.org>