"flip_done timed out" messages causing huge increase in boot time

[Daniel Kamil Kozar]

Hello.
After upgrading the kernel to 4.20, I noticed that the boot time
increased from about 5 seconds to 25 seconds. My machine is an Asus
K53SV with an Intel i7-2630QM, i.e. Sandy Bridge. I have an external
display connected to it via HDMI. If the display is unplugged when
booting the machine, the boot time stays at its old value.
The kernel log shows two messages like this :

[   15.747206] [drm:drm_atomic_helper_wait_for_flip_done
[drm_kms_helper]] *ERROR* [CRTC:39:pipe A] flip_done timed out
[   26.198968] [drm:drm_atomic_helper_wait_for_dependencies
[drm_kms_helper]] *ERROR* [CRTC:39:pipe A] flip_done timed out

Which are the only ones that don't appear in 4.19.2, where the boot
time was smaller. Bisecting the commits between these two versions
leads to commit 516a49cc19467e298d08a404f73a6e311f4548d1.

Let me know if you need more information.

Kind regards,
Daniel