viafb: suspend/resume for GPIOs

Be sure to re-enable GPIO lines on resume.  Users still have to
be sure to set them properly.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>