forcing "lock screen" does not power off the display anymore with 4.20

With 4.18 the display is put into power safe mode after DEFAULT_KEY_DPMS_SLEEP_AFTER seconds when the "lock screen" knob in the panel is used. With 4.20 the display just goes black, and remains powered on, until eventually the configured timeout in power manager triggers.

Since both are unrelated, one is for an idle system while the other is an pro-active choice, this looks like a regression.

In case the intention of commit apps/xfce4-screensaver@f959cd24 was to have a single place for DPMS changes, xfce4-powermanager may need to expose its xfpm_dpms_set_mode function for external users.

For some reason even xset dpms force off fails, the display is powered on again after a second, then the configured timeout in power manager triggers.