staging: olpc_dcon: move fb stuff info dcon_priv, and clean up fb handling

 - move fbinfo and ignore_fb_events into dcon_priv
 - add calls to {un,}lock_fb_info before calling fb_blank
 - fail to load the driver if there are no registered framebuffers

That last one fixes a potential oops, where if the dcon driver loads
without a framebuffer registered, fb_blank will end up being passed a
NULL (and will attempt to dereference it).

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>