Always call update_use_power() instead of directly setting the use_power var.

- This will let us react to the change appropriately.
- While we're here, lets define some constants so we can stop using bare numbers.
- Always use those constants when calling update_use_power()

code/modules/power/lighting.dm

@@ -320,10 +320,10 @@ var/global/list/light_type_cache = list()
 					on = 0
 					set_light(0)
 			else
-				use_power = 2
+				update_use_power(USE_POWER_ACTIVE)
 				set_light(brightness_range, brightness_power, brightness_color)
 	else
-		use_power = 1
+		update_use_power(USE_POWER_IDLE)
 		set_light(0)
 
 	active_power_usage = ((light_range * light_power) * LIGHTING_POWER_FACTOR)