KVM: x86/mmu: Refactor low-level TDP MMU set SPTE helper to take raw values

Refactor __tdp_mmu_set_spte() to work with raw values instead of a
tdp_iter objects so that a future patch can modify SPTEs without doing a
walk, and without having to synthesize a tdp_iter.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220226001546.360188-13-seanjc@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>