cpuidle/menu: stop seeking deeper idle if current state is deep enough

Obsolete commit 71abbbf8 (cpuidle: extend cpuidle and menu governor
to handle dynamic states) wanted to introduce dynamic C-states, but that
idea was dropped long ago.  The nonsense deeper C-state checking
remained, though.

Since both target_residency and exit_latency are longer for deeper
idle state, there's no need to waste CPU time on useless checks.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
Acked-by: Rik van Riel <riel@redhat.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>