drm/todo: mention i915 in the struct_mutex section

The i915 driver uses the struct_mutex, eventhough it does not use the
locked version of the drm_object_gem API.

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/20200515095118.2743122-4-emil.l.velikov@gmail.com