KVM: VMX: Use x2apic_mode to avoid RDMSR when querying PI state

Use x2apic_mode instead of x2apic_enabled() when adjusting the
destination ID during Posted Interrupt updates.  This avoids the costly
RDMSR that is hidden behind x2apic_enabled().

Reported-by: luferry <luferry@163.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210115220354.434807-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>