drm/docs: Discourage adding more to kms-properties.csv

Motivated by patch review.

The table is really hard to read in source form, hard to edit, and
we've moved away to more focused sections about specific features and
how they're exposed in properties.

Those sections can then more easily enumerate options, link to helper
functions and other parts of the docs. All things that get ugly real
fast in the docs.

Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219225356.24996-1-daniel.vetter@ffwll.ch