drm/doc: Polish docs for drm_mode_object

I figured an overview section here is overkill, and better
to just document the 2 structures themselves well enough.

v2: Review from Archit:
- Appease checkpatch in moved code.
- Spelling fixes in the kerneldoc.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-5-daniel.vetter@ffwll.ch