drm: Fix comment on mode parsing

Do not claim that there's a default mode in the video= option parser.
if no option string has been given, the parser does nothing.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209135509.7786-12-tzimmermann@suse.de