fbdev: Unexport fb_mode_option

There are no external users of fb_mode_option. Unexport the variable
and declare it static.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209135509.7786-7-tzimmermann@suse.de