panfrost: Preprocess shaders at CSO create time

panfrost: Preprocess shaders at CSO create time

Now the only passes that depend on the shader key can run late, so we can
preprocess ahead-of-time once and throw away the original shader. This reduces
the cost of shader variants, as well as deduplicates some lowering for
transform feedback shaders.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20906>