power_key: use 25Mhz devfreq when screen off when not on Linux 5.3

As on Linux 5.6 this is totally stable for me on Birch, I'd like to use 25Mhz
instead of 100 when the screen is off.

I ran "stress -c 2 -vm 2" while using the phone on the console and
turning on the screen, browsing the web and the system kept usable
the whole time.

With this, the device cools down more quickly when the screen is
off and improves battery time.

For Linux 5.3 (only), we keep the current behaviour as 25Mhz doesn't seem
to be stable there.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>