fbdev: Read video= option with fb_get_option() in modedb

Get the kernel's global video= parameter with fb_get_option(). Done
to unexport the internal fbdev state fb_mode_config. 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/20230209135509.7786-6-tzimmermann@suse.de