nouveau/nv50: Enable mesa/st alpha test lowering on nv50 with NIR.

nouveau/nv50: Enable mesa/st alpha test lowering on nv50 with NIR.

With TGSI, the driver allocates space for the alpha ref as a uniform and
adds a conditional discard to the shader.  We could either replicate that
with NIR, or just set the flag saying we need the shader lowering and get
the same thing.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16063>