drm/doc: drop struct_mutex references

There's little point in providing partial and ancient information about
the struct_mutex. Some drivers are using it, new ones should not.

As-it this only provides for confusion.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-5-emil.l.velikov@gmail.com