pan/bi: Disable lower_sub

For float, fixing up the modifier ourselves is easy. For int, we have a
dedicated isub instruction anyway.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4139>
2 files changed