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