drm/i915/selftests: Use preemption timeout on cleanup

Many selftests call igt_flush_test() on cleanup.  With default preemption
timeout of compute engines raised to 7.5 seconds, hardcoded flush timeout
of 3 seconds is too short.  That results in GPU forcibly wedged and kernel
taineted, then IGT abort triggered.  CI BAT runs loose a part of their
expected coverage.

Calculate the flush timeout based on the longest preemption timeout
currently configured for any engine.  That way, selftest can still report
detected issues as non-critical, and the GPU gets a chance to recover from
preemptible hangs and prepare for fluent execution of next test cases.

Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12061


Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241213190122.513709-2-janusz.krzysztofik@linux.intel.com