media: videobuf2: Fix doc comment

The documented struct member is called "planes" rather than "vb2_plane".
While at it, make the comments order follow struct members order.

Fixes: 2b141324

 ("media: vb2-core: Improve kernel-doc markups")
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>