drm/doc: Polish kerneldoc for drm_device.h

- Move all the legacy gunk at the bottom, and exclude it from
  kerneldoc.
- Documentation for the remaining bits.

v2: Fix typo (Sam).

Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111164048.29067-5-daniel.vetter@ffwll.ch