Drop the index param from do_cpuid_7_mask() and instead switch on the
entry's index, which is guaranteed to be set by do_host_cpuid().

No functional change intended.
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>