fbdev: Move option-string lookup into helper

Move the lookup of the option string into an internal helper. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209135509.7786-8-tzimmermann@suse.de