PM / cpuidle: Make ladder governor use the "disabled" state flag

For the mechanism introduced by commit cbc9ef02 (PM / Domains: Add
preliminary support for cpuidle, v2) to work with the ladder
governor, that governor should respect the "disabled" state flag
added by that commit.  Change the ladder governor accordingly.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>