drm/privacy-screen: Fix sphinx warning

Fix the following warning from "make htmldocs":

drivers/gpu/drm/drm_privacy_screen.c:392:
  warning: Function parameter or member 'data' not described in
  'drm_privacy_screen_register'

Fixes: 30598d9 ("drm/privacy_screen: Add drvdata in drm_privacy_screen")
Cc: Rajat Jain <rajatja@google.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207113307.346281-1-hdegoede@redhat.com

drivers/gpu/drm/drm_privacy_screen.c

@@ -379,6 +379,7 @@ static void drm_privacy_screen_device_release(struct device *dev)
  * drm_privacy_screen_register - register a privacy-screen
  * @parent: parent-device for the privacy-screen
  * @ops: &struct drm_privacy_screen_ops pointer with ops for the privacy-screen
+ * @data: Private data owned by the privacy screen provider
  *
  * Create and register a privacy-screen.
  *