=== fix applied to guest /usr/src ===
$ cd /usr/src && patch -p0 < /root/poc/fix.moduleparam.diff
Patching file sys/dev/drm/include/linux/moduleparam.h using Plan A:
Hunk #1 succeeded at 38.
$ cd /usr/src/sys/dev/drm/i915 && rm -f i915_params.o && make && make install
cc ... -c i915_params.c          (recompiled against fixed header)
cc -Wl,... -r -Wl,-d -o i915.ko ...
install -o root -g wheel -m 555   i915.ko /boot/kernel
$ reboot