drm: correct function name drm_legacy_ctxbitmap_flush()

Fix the following make W=1 kernel build warning:

  drivers/gpu/drm/drm_context.c:136: warning: expecting prototype for drm_ctxbitmap_flush(). Prototype was for drm_legacy_ctxbitmap_flush() instead
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210513071918.1728535-1-yangyingliang@huawei.com

drivers/gpu/drm/drm_context.c

@@ -124,7 +124,7 @@ void drm_legacy_ctxbitmap_cleanup(struct drm_device * dev)
 }
 
 /**
- * drm_ctxbitmap_flush() - Flush all contexts owned by a file
+ * drm_legacy_ctxbitmap_flush() - Flush all contexts owned by a file
  * @dev: DRM device to operate on
  * @file: Open file to flush contexts for
  *