fbdev: Unexport unlink_framebuffer()

There are no external callers of unlink_framebuffer() left. Make the
function an internal interface.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114125106.28347-4-tzimmermann@suse.de