Fixes the screens of unpowered PDAs disappearing (#22962)

As the title says, it has bothered me for a bit. Now every PDA base
sprite has a screen instead of relying on the overlays to add one on
them.
This _somewhat_ unfortunately resulted in the PDAs which had any alpha
(transparency) on their screens to no longer have them. No big loss.

From the changelog:
>  - bugfix: "The screens of unpowered PDAs no longer disappear"

Most likely should have done it it properly with overlays, but this also
works.

code/modules/modular_computers/computers/subtypes/dev_pda.dm

@@ -17,6 +17,12 @@
 	icon_state_unpowered = icon_state
 	icon_state_broken = icon_state
 
+/obj/item/modular_computer/handheld/pda/update_icon()
+	. = ..()
+	if(!enabled)
+		AddOverlays("off")
+		return
+
 /obj/item/modular_computer/handheld/pda/old
 	icon = 'icons/obj/modular_computers/pda_old.dmi'
 

code/modules/modular_computers/computers/subtypes/dev_wristbound.dm

@@ -18,6 +18,12 @@
 	icon_state_unpowered = icon_state
 	icon_state_broken = icon_state
 
+/obj/item/modular_computer/handheld/wristbound/update_icon()
+	. = ..()
+	if(!enabled)
+		AddOverlays("off")
+		return
+
 /obj/item/modular_computer/handheld/wristbound/attack_hand(mob/user)
 	if(ishuman(user))
 		var/mob/living/carbon/human/H = user