ANDROID: cpufreq: schedutil: maintain raw cache when next_f is not changed

Currently, the raw cache will be reset when next_f is changed after get_next_freq for correctness. However, it may introduce more cycles in those cases. This patch changes it to maintain the cached value instead of dropping it.

Bug: 159936782
Bug: 158863204
Signed-off-by: Wei Wang <wvw@google.com>
[dereference23: Backport to 4.14]
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>
Change-Id: I519ca02dd2e6038e3966e1f68fee641628827c82