EXYNOS: bugfix on retrieving old_index from freqs.old

The policy might have been changed since last call of target().
Thus, using cpufreq_frequency_table_target(), which depends on
policy to find the corresponding index from a frequency, may return
inconsistent index for freqs.old. Thus, old_index should be
calculated not based on the current policy.

We have been observing such issue when scaling_min/max_freq were
updated and sometimes cuased system lockups deu to incorrectly
configured voltages.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>