drm: Improve dumb callback docs

Noticed while reviewing a patch from Eric. Also add a todo for the
dumb_map_offset callbacks (it should be simple to do, but piles of
work). Plus fix up vbox, because vbox.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Fabio Rafael da Rosa <fdr@pid42.net>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181127091921.8325-1-daniel.vetter@ffwll.ch