radeon/radeon_vce: fix out of target bitrate in CBR mode (H.264)

StoneyRidge may not comply to required target bitrate
when generating H.264 stream in CBR mode.

Signed-off-by: Louis Li <Ching-shih.Li@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4384>