drm/doc: Helpers are not a Midlayer!

I'm devoting all my wrath to that fight, so don't misname it ;-)

v2: Make it clear that this section talks about kms helpers.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>