KVM: x86/mmu: Capture TDP level when updating CPUID

Snapshot the TDP level now that it's invariant (SVM) or dependent only
on host capabilities and guest CPUID (VMX).  This avoids having to call
kvm_x86_ops.get_tdp_level() when initializing a TDP MMU and/or
calculating the page role, and thus avoids the associated retpoline.

Drop the WARN in vmx_get_tdp_level() as updating CPUID while L2 is
active is legal, if dodgy.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200502043234.12481-11-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>