frontends/va: respect disable_bit_stuffing flag in AVC and HEVC rate control...

frontends/va: respect disable_bit_stuffing flag in AVC and HEVC rate control parameters, respect CAVLC/CABAC selection in AVC picture control parameters

Without this change, Mesa incorrectly assumes that the stream should always use CABAC entropy coding, whereas this is prohibited in baseline profile and should otherwise be controlled by the application through the h264 picture parameters. Mesa also incorrectly assumes that CBR encodings should always use filler data (aka "bit stuffing") to reach the target bitrate, even if the application has explicitly requested that this behavior be disabled through the rate-control misc parameter.