KVM: x86: use static_call_cond for optional callbacks

SVM implements neither update_emulated_instruction nor
set_apic_access_page_addr.  Remove an "if" by calling them
with static_call_cond().

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>