pan/bi: Model 3rd source for ATEST

pan/bi: Model 3rd source for ATEST

ATEST on Valhall takes atest-param as an explicit encoded source. The source
also exists on Bifrost, but it's implied in the encoding. Rather than treating
it as a "ghost" source, model it in the IR explicitly so the count is correct.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>