Fix volumetric toggle

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/VolumetricLighting.cs

 {
     public enum VolumetricLightingPreset
     {
+        Off,
         Normal,
         Ultra,
         Count
                 return 8;
             case VolumetricLightingPreset.Ultra:
                 return 4;
+            case VolumetricLightingPreset.Off:
+                return 0;
             default:
                 Debug.Assert(false, "Encountered an unexpected VolumetricLightingPreset.");
                 return 0;
                 return 64;
             case VolumetricLightingPreset.Ultra:
                 return 128;
+            case VolumetricLightingPreset.Off:
+                return 0;
             default:
                 Debug.Assert(false, "Encountered an unexpected VolumetricLightingPreset.");
                 return 0;