drm/doc: drop struct_mutex reference for drm_gem_object_free

The comment that struct_mutex must be held is misleading. It is only
required when .gem_free_object() is used.

Since that one is going with the next patches, drop the reference.

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/20200515095118.2743122-7-emil.l.velikov@gmail.com