mm-video: vidc: stability fixes in decoder

Synchronize access to extradata buffer which is allocated
and accessed in separate threads.
Do not unmap output handles in dynamic buffer mode when
buffers are freed (since they are unmapped already in FBD)

Bug: 11099331
Bug: 11366221

Signed-off: Leena Winterrowd <lenhardw@codeaurora.org>
Change-Id: I26d02c183d8496a52d4ded51c53f44eaf5e6e0b0