The DRM part is still not in the kernel tree and will not be there
until it stabilized. It won't happen anytime soon. So, consider using
the following patch in the meantime unless you don't want any hardware
accelerated 2D like EXA and Xvideo.

http://people.freebsd.org/~rnoland/drm-nouveau-062309.patch

Installing it should be as simple as below procedure provided you have
base sources in /usr/src.

    # cd /usr/src/sys
    # fetch http://people.freebsd.org/~rnoland/drm-nouveau-062309.patch
    # patch -sp0 -i drm-nouveau-062309.patch
    # cd modules/drm
    # make depend all install