staging: bcm2835-codec: Format changed should trigger drain

When a format changed event occurs, the spec says that it
triggers an implicit drain, and that needs to be signalled
via -EPIPE.

For BCM2835, the format changed event happens at the point
the format change occurs, so no further buffers exist from
before the resolution changed point. We therefore signal the
last buffer immediately.
We don't have a V4L2 available to us at this point, so set
the videobuf2 queue last_buffer_dequeued flag directly.

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