drm/privacy-screen: Fix sphinx warning

Fix the following warning from "make htmldocs":

drivers/gpu/drm/drm_privacy_screen.c:270:
 WARNING: Inline emphasis start-string without end-string.

Fixes: 8a12b170

 ("drm/privacy-screen: Add notifier support (v2)")
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://lore.kernel.org/r/20220207130407.389585-1-hdegoede@redhat.com
Link: https://patchwork.freedesktop.org/patch/msgid/20220207130407.389585-1-hdegoede@redhat.com