frontends/va: set vbv buffer size same as target bitrate in cbr mode

frontends/va: set vbv buffer size same as target bitrate in cbr mode

video buffering verifier (vbv) buffer size must be set same as
target bitrate to achieve constant bitrate.

Signed-off-by: Sajeesh Sidharthan <sajeesh.sidharthan@amd.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Reviewed-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19695>