media: vivid: make input std_signal per-input

Make the following properties per-input:

-Standard Signal Mode
-Standard

These properties need to be per-input in order to implement proper HDMI
(dis)connect-behavior, where the signal mode will be used to signify
whether or not there is an inpute device connected.

Signed-off-by: Johan Korsnes <johan.korsnes@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>