asahi: Put meta shader keys into the meta shader itself

asahi: Put meta shader keys into the meta shader itself

The hash table needs a key pointer with at least the lifetime of the
hash entry, which the key pointer we get does not have (since it is
stack-allocated by agx_build_meta). Copy it into the shader struct
itself and use that for the hash table.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20365>