drm/gem: use _unlocked reference in drm_gem_objects_lookup docs

Use the drm_gem_object_put_unlocked in the documentation for
drm_gem_objects_lookup. The locked version of the helper should be used
solely by people who know exactly what they are doing.

Should prevent issues like ones adddressed with the next patch.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515095118.2743122-3-emil.l.velikov@gmail.com