drm: Switch drm_plane_state to inline kerneldoc style

For consistency and to encourage more detailed documentation. While
doing this also beefed up a few of the comments, linking at least to
the setup function. Plus fixed all the hyperlinks.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-9-daniel.vetter@ffwll.ch