cpuidle: poll_state: Disregard disable idle states

When computing the limit of time to spend in the loop in poll_idle(),
use the target residency of the first enabled idle state deeper than
state 0 instead of always using the target residency of state 1.

This helps when state 1 is disabled for diagnostics, for instance.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>