omx: call gst_omx_buffer_unmap() when handling BUFFER_DONE

omx: call gst_omx_buffer_unmap() when handling BUFFER_DONE

When using a input buffer pool, the buffer may be released to the pool when
gst_omx_buffer_unmap() is called. We need to have buf->used unset at
this point as the pool may use it to check the status of the pool.

{Empty,Fill}BufferDone is called from OMX internal threads while
messages are handled from gst elements' thread. Best to do all this
when handling the message so we don't mess with OMX threads and keep
the original thread/logic split.

https://bugzilla.gnome.org/show_bug.cgi?id=796918