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 <[e-mail]>
Acked-by: Sam Ravnborg <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem.c

···
  * Returns:
  *
  * @objs filled in with GEM object pointers. Returned GEM objects need to be
- * released with drm_gem_object_put(). -ENOENT is returned on a lookup
+ * released with drm_gem_object_put_unlocked(). -ENOENT is returned on a lookup
  * failure. 0 is returned on success.
  *
  */