KVM: x86/tdp_mmu: Remove unused "kvm" of kvm_tdp_mmu_get_root()

The "struct kvm *kvm" parameter of kvm_tdp_mmu_get_root() is not used,
so remove it. No functional change intended.

Signed-off-by: Jinrong Liang <cloudliang@tencent.com>
Message-Id: <20220125095909.38122-5-cloudliang@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>