fix: screen not staying off (#1506)

When the screensaver is active and the screen is turned off due to DPMS
the unconditional call to self.screensaver.show wakes the screen up
again.
The check whether the screensaver is already active got lost in the
refactor.

Fixes #1504

screen.py

@@ -619,7 +619,8 @@ class KlipperScreen(Gtk.Window):
             self.set_dpms(False)
             return False
         elif state != functions.DPMS_State.On:
-            self.screensaver.show()
+            if not self.screensaver.is_showing():
+                self.screensaver.show()
         return True
 
     def wake_screen(self):