media: via-camera: call viafb_pm_unregister in remove()

The power management hooks were never unregistered, which caused a
crash when unloading the module.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>