I am comparing PLL values for a not flickering kernel:
Linux version 3.14.79-031479-generic
and version 4.15 (recent one):

Linux 4.15:
Flickering values
[    5.554557] [drm:radeon_compute_pll_avivo [radeon]] 122000 - 121980, pll
dividers - fb: 253.0 ref: 8, post 7
[  465.773119] [drm:radeon_compute_pll_avivo [radeon]] 122000 - 121980, pll
dividers - fb: 253.0 ref: 8, post 7
[  503.796178] [drm:radeon_compute_pll_avivo [radeon]] 122000 - 121980, pll
dividers - fb: 253.0 ref: 8, post 7

Linux version 3.14.79-031479-generic: no flickering
[4.250932] [drm:radeon_compute_pll_avivo], 12201, pll dividers - fb: 189.8 ref:
6, post 7
[  230.376566] [drm:radeon_compute_pll_avivo], 12201, pll dividers - fb: 189.8
ref: 6, post 7

122000 is about 10x 12201
I don't know what are these values, but it seems very different!