drm/doc: Consistent kerneldoc include order

First overview text (if there is any), then headers (since generally
you want to start out with the data structures), then all the other
stuff with functions.

Most of this is pre-shpinx, since with the old docbook only the
overview stuff was pulled in directly. Everything else was put in a
per-section index, so include order didn't really matter.

Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302151638.1882-4-daniel.vetter@ffwll.ch