fbdev/smscufx: Detect registered fb_info from refcount

Detect registered instances of fb_info by reading the reference
counter from struct fb_info.read. Avoids looking at the dev field
and prepares fbdev for making struct fb_info.dev optional.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-31-tzimmermann@suse.de