refactor(system): fix conflicts between desktop environment login managers

feat(system): re-add Totem to Gnome installs

system/modules/system/gpu/nvidia.nix

@@ -45,11 +45,11 @@ in
   config = lib.mkIf cfg.enable {
     assertions = [
       {
-        assertion = (cfg.hybrid.busIDs.nvidia != "");
+        assertion = (cfg.busIDs.nvidia == "");
         message = "You need to define a bus ID for your Nvidia GPU. To learn how to find the bus ID, see https://nixos.wiki/wiki/Nvidia#Configuring_Optimus_PRIME:_Bus_ID_Values_.28Mandatory.29.";
       }
       {
-        assertion = (cfg.hybrid.busIDs.intel != "" || cfg.busIDs.amd != "");
+        assertion = (cfg.busIDs.intel == "" && cfg.busIDs.amd == "");
         message = "You need to define a bus ID for your non-Nvidia GPU. To learn how to find your bus ID, see https://nixos.wiki/wiki/Nvidia#Configuring_Optimus_PRIME:_Bus_ID_Values_.28Mandatory.29.";
       }
     ];
@@ -73,8 +73,8 @@ in
         sync.enable = lib.mkIf cfg.hybrid.sync true;
 
         nvidiaBusId = cfg.hybrid.busIDs.nvidia;
-        intelBusId = cfg.hybrid.busIDs.intel;
-        amdgpuBusId = cfg.hybrid.busIDs.amd;
+        intelBusId = lib.mkIf (cfg.hybrid.busIDs.intel != "") cfg.hybrid.busIDs.intel;
+        amdgpuBusId = lib.mkIf (cfg.hybrid.busIDs.amd != "") cfg.hybrid.busIDs.amd;
       };
     };
   };