Christian König posted an explanation of the PLL divider values at
https://bugzilla.kernel.org/show_bug.cgi?id=91861#c12 (another "no screen after
3.15" bug report)

The various fixes adjust the divider value limits slightly for different
displays. The basic formula is commented in the radeon_compute_pll_avivo
function:

        dot_clock = (ref_freq * feedback_div) / (ref_div * post_div)

So by adjusting the limits of those values you can find something that works
for your laptop display. But I don't know which solution is technically correct
- if you don't get a reply here you could try emailing Christian König and
asking.