staging/bcm2835-codec: Do not send buffers to the VPU unless streaming

With video decode we now enable both input and output ports on
the component. This means that buffers will get passed to the VPU
earlier than desired if they are queued befoer STREAMON.

Check that the queue is streaming before sending buffers to the VPU.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>