drm/doc: Move legacy kms helpers to the very end

We don't want people to accidentally stumble over there.

Also rename the plane helpers to legacy plane helpers. After Ville's
patch to make the clipping helper atomic and move it to
drm_atomic_helper.c there's nothing left in there that should be
useful for modern drivers.

v2: Laurent had a few questions around how state is added to
drm_atomic_state, tried to clarify that. And spotted another sentence
where the docs suggested subclassing.

v3: Small polish (Alex).

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171214203054.20141-6-daniel.vetter@ffwll.ch