drm/edid: make drm_edid_duplicate() safe to call with NULL parameter

It's a bit tedious to check for NULL before calling
drm_edid_duplicate(). Make it handle NULL parameter graciously.
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1ae8923d39a5abd0260fdf7f9cc54c5e046e70d3.1685437501.git.jani.nikula@intel.com

drivers/gpu/drm/drm_edid.c

@@ -2881,6 +2881,9 @@ EXPORT_SYMBOL(drm_edid_read_switcheroo);
  */
 struct edid *drm_edid_duplicate(const struct edid *edid)
 {
+	if (!edid)
+		return NULL;
+
 	return kmemdup(edid, edid_size(edid), GFP_KERNEL);
 }
 EXPORT_SYMBOL(drm_edid_duplicate);