Created attachment 96962 [details]
xrandr 1920x1080x23.98 working 76564 removed

OK.  I finally got the patch removed and rebuilt (thanks to Peter's tips). 
Xrandr 23.98 now works as expected.  No blank screen:
[   99.956862] [drm:radeon_compute_pll_avivo], 14818, pll dividers - fb: 32.6
ref: 2, post 11

With the patch "linux-996-drm-radeon-rework-finding-display-PLL-numbers.patch"
it failed.  I will upload this patch, that I removed from the source folder.  I
tried the 3 line patch x2, when it was present but I am not sure it took (built
to fast to see on the screen).  So I will add the 3 changes to non-working
patch and rebuild, next. and dmesg it for pll vals.