zink: unspaghettify some program update code

zink: unspaghettify some program update code

the initial implementation of optimal_keys was added inline, but really
it's an entirely different codepath. by separating these out, it makes the
code more readable, and it also allows for slightly better optimization of
of the optimal_keys codepath

~4-5% improvement for drawoverhead -test 7

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18786>