drm: use the right function name in documentation

There is no late_unregister(), it looks like the comment meant
late_register(). Also fix a typo while at it.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476032820-3275-1-git-send-email-notasas@gmail.com