drm/docs: Document "scaling mode" property better

Move it out of the csv dungeon.

While at it add the missing link to the helper functions for setting
up the "panel rotation" property.

Also OCD how we list enum property values and their corresponding docs.
Going for a nest definition list seams cleanest, no need for also making
it an uordered list.

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219225356.24996-3-daniel.vetter@ffwll.ch