omxbufferpool: fix race when releasing input buffers

Guillaume Desmottes requested to merge gdesmott/gst-omx:input-buffer-race into master

If buffers were released from the pool while gst_omx_video_enc_handle_frame() was waiting for new buffers, gst_omx_port_acquire_buffer() was never awaken as the buffers weren't released through OMX's messaging system.

GQueue isn't thread safe so also protect it with the lock mutex.

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