pan/bi: Add validation for nr_srcs/nr_dests

pan/bi: Add validation for nr_srcs/nr_dests

Now that we set nr_srcs/nr_dests accurately, assert as much in the validator.
This pass will be deleted later in the series, but having it here is expected to
be useful for bisection, in case there are cases missed. Certainly running the
CTS at this point in the series is helpful to prove completeness of the
beginning of this series.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>