> As far as I understand; 3.15.0-rc2-00077-g76e7745 is a later revision (good)
> than 3.15.0-rc2-00069-g1aae31c (bad)

This is not correct. The 77/69 does not imply a linear ordering because of
forks:

$ git merge-base --is-ancestor 3.15.0-rc2-00069-g1aae31c
3.15.0-rc2-00077-g76e7745; echo $?
1

Trust git ;-)

> c2fb3094669a3205f16a32f4119d0afe40b1a1fd is the first bad commit

Not familiar with this code, but from the patch the PLL values are printed out:

         DRM_DEBUG_KMS("%d - %d, pll dividers - fb: %d.%d ref: %d, post %d\n",
                      freq, *dot_clock_p * 10, *fb_div_p, *frac_fb_div_p,
                      ref_div, post_div);

So suggest enabling debug log and compare those two lines from a working and
non-working kernel.

It should also be trivial to checkout a recent tag and revert the bad commit
(there is a conflict but just delete the avivo_get_fb_ref_div function to
resolve it).