fbdev: Support NULL for name in option-string lookup

Ignore the per-driver video options if no driver name has been
specified to fb_get_option(). Return the global options in this
case.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209135509.7786-4-tzimmermann@suse.de