frontends/va: remove private member and update target buffer

frontends/va: remove private member and update target buffer

use update_decoder_target to update the target buffer to
let decoder obtain correct reference frame.

remove the previous logic which failed to update reference
info in time.

fixes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8996
fixes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8387
Cc: mesa-stable
Reviewed-by: Sil Vilerino <sivileri@microsoft.com>
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23061>