List input coercion not work with enum provided as variable

### What happened?

When you send query graphql is able to coerce single value to an array:

```graphql
# WOKS!
query vessels {
vessels(shipType: [CAR_CARRIER]) {
nodes {
id
}
}
}
```

```graphql
# WORKS!
query vessels {
vessels(shipType: CAR_CARRIER) {
nodes {
id
}
}
}
```

But when enum value submitted via variables, the coercion does not work:

```graphql
# FAILS!
query vessels($shipType: [ShipType!]) {
vessels(shipType: $shipType) {
nodes {
id
}
}
}
```

with variables:
```json
{
"shipType": "CAR_CARRIER"
}
```

with message `"enums must be strings"`

### What did you expect?

Array coercion works for variables.

### Minimal graphql.schema and models to reproduce

```graphql
type Query {
vessels(shipType: [ShipType!]!): [Vessel]
}

enum ShipType {
CAR_CARRIER
}

```

### versions
- `gqlgen version` - v0.14.0
- `go version` - go 1.17.2