pan/bi: Use safe helpers to remove srcs/dests

pan/bi: Use safe helpers to remove srcs/dests

Changing I->nr_srcs or I->nr_dests directly is generally unsafe, but the special
case of removing sources/destinations at the end is safe. Add and use helpers to
wrap this operation simplifying the remaining code audit before we can
dynamically allocate sources/destinations.

At this point in the series, nothing modifies I->nr_dests after allocation
except these helpers, so destinations should be safe to make dynamic. There's a
bit more work needed for sources.

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