Ok. I did everything like you have suggested. But maybe I did some mistake. So,I want to make a recap :
0) I've used the phisycal installation of netBSD to do everything
1) I have uncommented these lines in sys/arch/amd64/conf/GENERIC:
options DRM_MAX_RESOLUTION_HORIZONTAL=1920 # Limit DRM size in horizontal dimension
options DRM_MAX_RESOLUTION_VERTICAL=1080 # Limit DRM size in vertical dimension
2) I've recreated the drm devices with :
cd /dev
sh ./MAKEDEV drm0 drm1 drm2
3) recompiled tools and kernel with the following commands :
./build.sh -U -j14 -m amd64 -O /home/mario/Desktop/Dati/obj -T /home/mario/Desktop/Dati/tools tools
./build.sh -U -j14 -m amd64 -O /home/mario/Desktop/Dati/obj -T /home/mario/Desktop/Dati/tools kernel=GENERIC
3.1) copied the netbsd kernel from /home/mario/Desktop/Dati/obj/sys/arch/amd64/compile/GENERIC/netbsd to / with the following command 😀
cp /home/mario/Desktop/Dati/obj/sys/arch/amd64/compile/GENERIC/netbsd /
4) I've edited the file boot.cfg like this 😀
nano /boot.cfg
menu=Boot normally:rndseed /var/db/entropy-file;gop 4;boot
menu=Boot single user:rndseed /var/db/entropy-file;boot -s
menu=Drop to boot prompt:prompt
userconf=disable nouveau*
#userconf=disable i915drmkms*
default=1
timeout=5
clear=1
5.1) I don't know if it useful,but I did an additional try,trying to boot netBSD with bhyve and trying to capture the early boot messages because I wanted to know what's happened as soon as netbsd booted,to understand the reason why the screen turns off and I've captured the messages that u see on the picture below

6) I've used this xorg.conf file,saved on : /etc/X11/xorg.conf.d
Section "Device"
Identifier "Card0"
Driver "modesetting"
EndSection
what else ? nothing is changed. At some point the screen turns off and I can't use netBSD anymore.