zink: zero out zink_render_pass_state

zink: zero out zink_render_pass_state

Since zink_render_pass_state is used as a hash-key, the entire struct gets
compared. This means we don't want any uninitialized padding in there, or
else we risk getting false negatives. This has led to issues on macOS builds.

So let's zero out the struct before we start filling it out.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4212>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4212>