arm64/vmm: Move the vgic_max_cpu_count() check

vm_alloc_vcpu() is called quite frequently, and we don't need to apply
the vgic limit unless we're actually allocating a vcpu structure for the
first time.

No functional change intended.

Reviewed by: andrew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D53580