frontends/va: Only set VP9 segmentation fields when segmentation is enabled
What does this MR do and why?
frontends/va: Only set VP9 segmentation fields when segmentation is enabled
Workaround for ffmpeg setting segmentation_update_map to 1 with
segmentation_enabled == 0.
Fixes decoding sample from https://github.com/mpv-player/mpv/issues/13533
Cc: mesa-stable