kvm: arm/arm64: Remove spurious WARN_ON

On a 4-level page table pgd entry can be empty, unlike a 3-level
page table. Remove the spurious WARN_ON() in stage_get_pud().

Acked-by: Christoffer Dall <cdall@kernel.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>