$ kldload drm
$ cd /tmp/df4919 && make
Warning: Object directory not changed from original /tmp/df4919
dragonfly -> /usr/src/sys
machine_base -> /usr/src/sys/platform/pc64
machine -> /usr/src/sys/platform/pc64/include
cpu_base -> /usr/src/sys/cpu/x86_64
cpu -> /usr/src/sys/cpu/x86_64/include
cc  -O2 -pipe -I/usr/src/sys/dev/drm/include -I/usr/src/sys/dev/drm/include/drm -I/usr/src/sys/dev/drm/include/uapi -include /usr/src/sys/dev/drm/kconfig.h -DKBUILD_MODNAME=\"df4919\"  -D_KERNEL -Wall ... -Werror -DKLD_MODULE -nostdinc ... -c df4919.c
cc -Wl,--build-id=sha1 -nostdlib -Wl,--hash-style=sysv   -r -Wl,-d -o df4919.ko df4919.o
== full untrimmed output of the final successful build (-Werror clean, 0 errors)