KVM: arm64: Consolidate dist->ready setting into kvm_vgic_map_resources()

dist->ready setting is pointlessly spread across the two vgic
backends, while it could be consolidated in kvm_vgic_map_resources().

Move it there, and slightly simplify the flows in both backends.

Suggested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>