drm: Update connector documentation

- Shuffle docs from drm-kms.rst into the structure docs where it makes
  sense.
- Put the remaining bits into a new overview section.

One thing I've changed is around probing: Old docs says that you
_must_ use the probe helpers, which isn't correct. Helpers are always
optional.

v2: Review from Sean.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-17-git-send-email-daniel.vetter@ffwll.ch