Dashboard cells off screen are not updated when variables change

Only cells visible on screen are reloaded for variable changes on dashboards. The way I've observed this is having multiple hosts reporting into a single bucket but I'd expect it to be the same when switching between buckets.

__Steps to reproduce:__

1. Install telegraf on two hosts and have them report in to a single bucket
2. Create a host variable of type query
```
import "influxdata/influxdb/schema"`

schema.tagValues(bucket: v.bucket, tag: "host")
```
3. Import the stock system dashboard from the template, add the following filter to two of the cells/graphs, drag one of them below the viewable area.
`|> filter(fn: (r) => r["host"] == v.host)`
4. Change between the two host variables from the drop down and observe the two graphs by scrolling up and down

__Expected behavior:__
Both graphs to change which host is filtered to when the host variable is updated

__Actual behavior:__
Only the graph currently on the screen updates - the other is left filtering to the previous host

__Environment info:__

* System info: Linux 5.4.0-66-generic x86_64
* InfluxDB version: InfluxDB 2.0.4 (git: 4e7a59bb9a) build_date: 2021-02-08T17:47:02Z
* Other relevant environment details: Ubuntu 20.04

Reproduce the dashboard with two hosts, a host query variable, and one filtered graph below the visible area. Start by tracing how dashboard cells reload when the variable changes, then verify that both visible and off-screen graphs switch to the selected host after the change.

Written by the indexing model from the issue text.