viafb: fix proc entry removal

Trying to remove unregistered proc entries became painful and is
useless anyway. So remove the removal of an entry that was never
registered and duplicate the logic for one which is added
conditionally. Additionally move the removal above releasing fb_info
as we still need the information.
This prevents tainting the kernel by the procfs warn on and
avoiding access to already freed memory is probably also a good idea.
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>