omxbufferpool: fix race when releasing input buffers

omxbufferpool: fix race when releasing input buffers

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.