i915 fastset failing on XPS 13 7390

On a XPS 13 7390 (i7-10510U) on current Ubuntu (amd64) the plymouth (git snapshort, spinner/bgrt theme) screen flickers, which seems to be due to fastset failing

I'm adding a dmesg log from a boot with drm.debug setdmesg.txt

The log has those errors

[    4.314789] [drm:pipe_config_mismatch [i915]] fastset mismatch in dp_m_n (expected tu 64 gmch 8307052/8388608 link 346127/524288, or tu 0 gmch 0/0 link 0/0, found tu 64, gmch 3322820/8388608 link 276901/1048576)
[    4.314823] [drm:pipe_config_mismatch [i915]] fastset mismatch in port_clock (expected 216000, found 540000) 

I've tested on a 5.6-rc5 mainline build from http://kernel.ubuntu.com/~kernel-ppa/mainline/ and the issue is still there