fill-pattern with step zoom does not show up on load

Nobody has claimed this yet.

The `fill-pattern` property doesn't appear to be working properly when it changes based on a step zoom function like this:

```
[
"step",
["zoom"],
"attraction-15",
14,
"american-football-11"
]
```

**mapbox-gl-js version**: 1.10.1

**browser**: Mac OS 10.14.4 Chrome 81.0.4044.138

### Steps to Trigger Behavior

1. Have a style with a `fill-pattern` layer that has a step based on zoom (see above)
2. Load map just past step threshold (14.1)
3. Pattern doesn't show

Also when you start further zoomed-out than threshold (13.9) then zoom in and cross the step threshold, the new pattern does not show up at first.

### Link to Demonstration

https://jsbin.com/qupayegoza/edit?html,output

### Expected Behavior

The pattern should show up when the map first loads and when you cross the step threshold

### Actual Behavior

No pattern shows up when map first loads until you zoom in then out or out then in.

Start with the linked JSBin reproduction and the fill-pattern style using the step zoom expression. Reproduce both initial-load cases and the threshold crossing at zoom 14, then trace the fill-pattern handling in mapbox-gl-js. Done means the pattern appears on load and updates when crossing the step threshold without requiring an additional zoom.

Written by the indexing model from the issue text.