I've build a 4.15.5 kernel incl. your patch and booted with display connected
to hdmi.
First boot hang.
In journalctl I found
Feb 24 11:29:28 linux kernel: [drm:drm_atomic_helper_wait_for_flip_done
[drm_kms_helper]] *ERROR* [CRTC:42:crtc-0] flip_done timed out
(I'll attach journalctl-boot-4-15-5-with-patch-hdmi-hang.txt)

Second try booted into gui (kde login). The colors where still wrong. But now
it was possible to turn of the screen and on again. The screen resolution
stayed the same but the colors still were wrong.
(I'll attach dmesg-4-15-5-with-patch-hdmi.txt and
Xorg.0.log-4.15.5-with-patch-hdmi.txt)

I'll stay with the dvi connected monitor.