media: venus: Helper function for dynamically updating bitrate

Move the dynamic bitrate updating functionality to a separate function
so that it can be shared.

No functionality changes.

Signed-off-by: Fritz Koenig <frkoenig@chromium.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>