venc: tweak bitrate calculation
venc: tweak bitrate calculation

So that we get 3-4 mbps.

Signed-off-by: Felipe Contreras <de77a8b9cd239b204c19c52ed62e48ab47243362@gmail.com>

community-ssu/gst-dsp,community-ssu/gst-dsp,felipec/gst-dsp,felipec/gst-dsp,community-ssu/gst-dsp