The following query returns a list of domains under an address but there's no way to tell which one is the primary ENS name:
domains(
where: {
resolvedAddress: "0x983110309620d911731ac0932219af06091b6744"
}
) {
name
}
On app.ens.domains it shows the address has a primary ENS name:
Primary ENS Name (reverse record): brantly.eth