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

drivers/gpu/drm/drm_gem.c

@@ -965,7 +965,6 @@ EXPORT_SYMBOL(drm_gem_object_release);
  * @kref: kref of the object to free
  *
  * Called after the last reference to the object has been lost.
- * Must be called holding &drm_device.struct_mutex.
  *
  * Frees the object
  */