kvm: arm64: vgic-v3: Introduce vgic_v3_free_redist_region()

To improve the readability, we introduce the new
vgic_v3_free_redist_region helper and also rename
vgic_v3_insert_redist_region into vgic_v3_alloc_redist_region

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210405163941.510258-8-eric.auger@redhat.com