radv: Add pipeline type.

I want to keep pointers that need to be freed in the union and need
to figure out the type a destruction time. This seems more reliable
than checking the shader array in case we destroy mid-creation (i.e.
on failure).

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12592>