Linux DRM/KMS graphics driver subsystem.

To import a new version, follow the instructions at the top of
prepare-import.sh.  TODO(riastradh): Elaborate.

prepare-import.sh       script to add RCS tags and stuff
drm/
  drm2netbsd            script to generate files.drmkms draft
nouveau/
  nouveau2netbsd        script to uniquify nouveau files and emit files.nouveau

TODO(riastradh): Uniquify the radeon and amdgpu file names like we do
with nouveau, because currently radeon and amdgpu fight over some
files.