Braswell is supported (though if ALL processors of the series are supported isn't guaranteed)
Have you installed a Xorg already and start some DE through a desktop login manager?
If that's the case, then you should disable loading of the
i915kms (
graphics/drm-next-kmod)driver and the start of the desktop manager in
/etc/rc.conf. Then reboot.
When FreeBSD is up, load the driver by hand
# kldload /boot/modules/i915kms.ko
and see what happens. You should see messages from the driver attaching and registering the video outputs and stuff. The screen shouldn't go black.
Then start your desktop manager. For
x11/slim that would be
service slim onestart
(kdm and gdm for KDE and Gnome I think, but not sure)
If the screen goes blank now, I bet you have something wrong in your
xorg.conf.
The easiest way to check that, is to simply move the file to some other place and let Xorg autoconfigure itself when it starts. Works very well.