piglit.spec.egl_mesa_configless_context.basic regression

This test was flaky on intel/iris, but has started failing on all runs beginning with:

bad 04826cd9fc83365e99db3a491254a8d5a6eaac63
Author:     Adam Jackson <ajax@redhat.com>

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>

reproduce with:

piglit run -p gbm -t egl_mesa_configless_context quick /tmp/

Fails with:

warning: couldn't find a 16-bit config with no depth buffer
Binding incompatible surfaces together unexpectedly succeeded