While the workaround with power_dpm_force_performance_level/pp_dpm_mclk works
for me, I found another way to stop the flickering, which I mention here in
case it helps debugging.

My monitor has a native resolution of 1920x1200 at 75Hz or 60Hz through DP, and
60Hz through HDMI.  It supports Freesync through DP, but I have that disabled. 
I'm using the 4.19 kernel.

When through a DP cable, if I switch the resolution to 1920x1080 at 60Hz, and
then revert back to the native 1920x1200 at 75Hz, the flickering stops for
good.  If I turn off and on the monitor, everything is still good, but if I
reboot or do a sleep and wakeup of the computer, then the flickering returns,
that is until I switch the resolution again.

Through HDMI, I don't get the flickering on the native resolution, possibly
because I get this max resolution at 60Hz.  But note that through DP, changing
to 60Hz alone doesn't seem to help.

Also, through HDMI, I can make the flickering start, if I switch to 1440x900 at
75Hz, and then it only stops if I switch to 1920x1080 at 60Hz and then back to
native.

Finally, after I got the flickering stop with the resolution switch, I tried
watching the pp_dpm_mclk, and I could see it changing values very fast, similar
to when I had the flickering problem.