vt: Fix up unregistration of vt drivers

vt: Fix up unregistration of vt drivers

A bunch of issues:
- We should not kick out the default console (which is tracked in
  conswitchp), so check for that.
- Add better error codes so callers can differentiate between "something
  went wrong" and "your driver isn't registered already". i915 needs
  that so it doesn't fall over when reloading the driver and hence
  vga_con is already unregistered.
- There's a mess with the driver flags: What we need to check for is
  that the driver isn't used any more, i.e. unbound completely (FLAG_INIT).
  And not whether it's the boot console or not (which is the only one
  which doesn't have FLAG_MODULE). Otherwise there's no way to kick
  out the boot console, which i915 wants to do to prevent havoc with
  vga_con interferring (which tends to hang machines).

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>