drm: Cleanups after drmm_add_final_kfree rollout

A few things:
- Update the example driver in the documentation.
- We can drop the old kfree in drm_dev_release.
- Add a WARN_ON check in drm_dev_register to make sure everyone calls
  drmm_add_final_kfree and there's no leaks.

v2: Restore the full cleanup, I accidentally left some moved code
behind when fixing the bisectability of the series.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-20-daniel.vetter@ffwll.ch