drm/doc: demote old doc-comments in drm.h

Sphinx doesn't like old doc-comments in drm.h and generates warnings
like:

    ./include/uapi/drm/drm.h:87: warning: cannot understand function prototype: 'struct drm_clip_rect '
    ./include/uapi/drm/drm.h:97: warning: cannot understand function prototype: 'struct drm_drawable_info '
    ./include/uapi/drm/drm.h:105: warning: cannot understand function prototype: 'struct drm_tex_region '
    ...

Demote these to regular comments, because converting all of them is
quite a lot of work (also requires documenting all of the struct fields
for instance). Also many of these structures aren't really used by
modern user-space.

We can easily convert these remaining old comments to Sphinx style on a
one-by-one basis.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222133524.160842-5-contact@emersion.fr