KVM: x86/mmu: Merge TDP MMU put and free root

kvm_tdp_mmu_put_root and kvm_tdp_mmu_free_root are always called
together, so merge the functions to simplify TDP MMU root refcounting /
freeing.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-5-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>