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