fbdev: Warn in hot-unplug workaround for framebuffers without device

A workaround makes fbdev hot-unplugging work for framebuffers without
device. The only user for this feature was offb. As each OF framebuffer
now has an associated platform device, the workaround hould no longer
be triggered. Update it with a warning and rewrite the comment. Fbdev
drivers that trigger the hot-unplug workaround really need to be fixed.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Suggested-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220419100405.12600-3-tzimmermann@suse.de