drm/doc: Light drm-kms-helper.rst cleanup

drm/doc: Light drm-kms-helper.rst cleanup

- Move the common vtable stuff to the top
- Move "Tile Group" to a more appropriate heading level
- Throw away the old intro for the crtc helpers (it's entirely stale,
  e.g. helpers have become modular years ago), and replace it with a
  general intro about the motivation behind helpers.
- Reorder helpers to group them together a bit better, and explain
  that grouping in the intro.
- Make sure the introductory DOC section is always first.

v2:
- Remove bogus files accidentally added (Sean).
- Spelling fixes (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/1471034937-651-2-git-send-email-daniel.vetter@ffwll.ch