drm/i915: No-video regression on Chromebook with Linux 6.5

An Acer Chromebook C740 with Intel Corporation HD Graphics (rev 08), according to lspci, which worked fine with Linux 6.4.x fails to initialize the built-in flat panel with Linux 6.5.1

What I think is the relevant part of the dmesg output is this (full output in dmesg-drm-tip.txt):

[   13.169944] i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:owns=io+mem
[   13.192997] [drm] Initialized i915 1.6.0 20201103 for 0000:00:02.0 on minor 0
[   13.193747] ACPI: video: Video Device [GFX0] (multi-head: yes  rom: no  post: no)
[   13.194854] input: Video Bus as /devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/LNXVIDEO:00/input/input3
[   13.195004] snd_hda_intel 0000:00:1b.0: bound 0000:00:02.0 (ops i915_audio_component_bind_ops [i915])
[   13.207950] i915 0000:00:02.0: [drm] Cannot find any crtc or sizes
[   13.240357] i915 0000:00:02.0: [drm] Cannot find any crtc or sizes
[   13.244087] i915 0000:00:02.0: [drm] Cannot find any crtc or sizes

Bisecting points to cfe5bdfb27fa234505e96f7775d32415ed705f94 (author @vsyrjala) as the culprit.

The symptoms are the same with the current drm-tip (20f073d076ba818ce9109eed6d11e63c287583d2), the dmesg output of a boot, with drm.debug=0xe log_buf_len=4M on the kernel command line, is in dmesg-drm-tip.txt.

I'm perfectly happy to assist in further debugging.

Cheers,

Frej