drm/doc: add WARNING for drm_device::struct_mutex

The mutex should be used, only by legacy drivers. Add a big warning to
deter people from using it.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-6-emil.l.velikov@gmail.com