rpicamsrc: Implement dynamic bitrate update
rpicamsrc: Implement dynamic bitrate update

Use mmal_port_set_parameter_uint32 to update the encoder
bitrate.

Fixes https://github.com/thaytan/gst-rpicamsrc/issues/60

pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good