From IRC:
<Anssi> agd5f: you might've seen already, but the 23.976 Hz pixel clock 
74175.824175 is rounded up to 74176 in Ville's commit, but
r600_hdmi_predefined_acr[] table contains 74175 so it does not match
<Anssi> not sure why the calculated value would not work, though... probably
for the same reason the recommended value is not the calculated value, but I
don't understand enough to know why