drm: switch drm_plane to inline comments

And use that opportunity to polish the kernel doc all around:
- Beef up some of the documentation.
- Intro text for drm_plane and better links
- Fix all the hyperlinks!

v2: Fix linebreaks.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-10-daniel.vetter@ffwll.ch