KVM: x86/mmu: Refactor is_tdp_mmu_root into is_tdp_mmu

This change simplifies the call sites slightly and also abstracts away
the implementation detail of looking at root_hpa as the mechanism for
determining if the mmu is the TDP MMU.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210617231948.2591431-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>