Thanks for the suggestion, but it appears the problem was earlier. I built an up-to-date kernel with the four files downgraded to the versions prior to your change, but the result was the same - the system working fine but with a black screen. Xorg also starts, judging by the processes present, but does not alight the screen. This is with the following versions:
---
➜ conf # head -1 /usr/src/sys/external/bsd/drm2/include/drm/drmfb.h
/* $NetBSD: drmfb.h,v 1.1 2015/03/05 17:50:41 riastradh Exp $ */
➜ conf # strings /netbsd | grep \$NetBSD | egrep intelfb.c\|nouveaufb.c\|drmfb.c
$NetBSD: drmfb.c,v 1.2 2015/11/09 23:11:18 jmcneill Exp $
$NetBSD: intelfb.c,v 1.12 2015/03/05 17:56:39 riastradh Exp $
$NetBSD: nouveaufb.c,v 1.3 2015/10/17 12:02:44 jmcneill Exp $
Regards,
Chavdar