drm: Fix typo in drm_event_reserve_init() kerneldoc

drm_event_reserve_init_locked() is the correct function to call when
already holding the dev->event_lock lock.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161215113602.27966-1-thierry.reding@gmail.com

drivers/gpu/drm/drm_fops.c

@@ -622,7 +622,7 @@ EXPORT_SYMBOL(drm_event_reserve_init_locked);
  * kmalloc and @p must be the first member element.
  *
  * Callers which already hold dev->event_lock should use
- * drm_event_reserve_init() instead.
+ * drm_event_reserve_init_locked() instead.
  *
  * RETURNS:
  *