docs: remove incorrect drivers from extension

docs: remove incorrect drivers from extension

This extension isn't wired up in Gallium, so there's just no way a
Gallium driver like Panfrost exposes it.

While there were support in i965 for this for the cancelled Broxton GPU,
thre's no such support in the Iris driver. And since Broxton has been
cancelled, it's unlikely to be wired up any time soon.

Fixes: da23a31726f ("docs/features: Update ASTC entries for Panfrost")
Acked-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15145>