gallium/cso_hash: remove another layer of pointer indirection

gallium/cso_hash: remove another layer of pointer indirection

Convert this:

    struct cso_hash {
       union {
          struct cso_hash_data *d;
          struct cso_node      *e;
       } data;
    };

to this:

    struct cso_hash {
       struct cso_hash_data data;
       struct cso_node      *end;
    };

1) data is not a pointer anymore.
2) "end" points to "data" and acts as the end of the linked list.
3) This code is still crazy.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3829>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3829>