KVM: x86/mmu: Replace open coded usage of tdp_mmu_page with is_tdp_mmu_page()

Use is_tdp_mmu_page() instead of querying sp->tdp_mmu_page directly so
that all users benefit if KVM ever finds a way to optimize the logic.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221012181702.3663607-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>