KVM: x86/mmu: Refactor TDP MMU iter need resched check

Refactor the TDP MMU iterator "need resched" checks into a helper
function so they can be called from a different code path in a
subsequent commit.

No functional change intended.

Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240823235648.3236880-4-dmatlack@google.com


[sean: rebase on a swapped order of checks]
Signed-off-by: Sean Christopherson <seanjc@google.com>