egl: Factor some common terminate cleanup up to common code

egl: Factor some common terminate cleanup up to common code

It's a little difficult to see from the diff, but this is effectively
the same calling sequence as before, and more importantly it means the
backend only cleans up backend state rather than needing to call up to
the core.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18824>