zink: fix `final_hash` update in `zink_gfx_program_update`

zink: fix `final_hash` update in `zink_gfx_program_update`

The logic that updates `ctx->gfx_pipeline_state.final_hash` assumed that
the program is replaced. It is supposed to xor `final_hash` with the
hash first and then with the new hash however when the program is
updated it end up xor-ing the new hash twice so it does nothing.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Fixes: 15450d2c2e2 ("zink: incrementally hash all pipeline component hashes")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21925>