drm/bridge and lockup on boot

[Dominik Brodowski]

On my old Dell XPS 13 laptop with

00:00.0 Host bridge: Intel Corporation Broadwell-U Host Bridge -OPI (rev 09)
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 5500 (rev 09) (prog-if 00 [VGA controller])

booting 5.6-rc1 and -rc2 fails after the dmesg line

	fb0: switching to inteldrmfb from simple

while the next lines should be something like (v5.5):

	Console: switching to colour dummy device 80x25
	i915 0000:00:02.0: vgaarb: deactivate vga console
	[drm] ACPI BIOS requests an excessive sleep of 25000 ms, using 1500 ms instead
	[drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
	[drm] Driver supports precise vblank timestamp query.
	i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:owns=io+mem

A git bisect lead to

	commit b86d895524ab ("drm/bridge: Add an ->atomic_check() hook")

as the first bad commit, as unlikely as that sounds. f7619a58ef92 is good,
as is bf046007641a, and 3cacb2086e41 is definitely broken on my setup.
Any ideas?

Oh, and this might be the same issue as reported here:

	https://lore.kernel.org/lkml/99fb887f-4a1b-6c15-64a6-9d089773cdd4@xxxxxxx/

though I do not see such a warning, but nothing new once the line "fb0: switching
to inteldrmfb from simple" is printed.

Thanks,
	Dominik