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: 30598d92 ("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