media: rcar-vin: Store scaler in a function pointer

The scaler implementation is different between the VIN generations, and
not all SoCs have a scaler. Currently only Gen2 scalers are supported.

Prepare to add support for more scalers by storing the setup in a
function pointer initialized at probe time. While at it move call site
to after, instead of before, the generic capture setup, this have no
effect on the Gen2 scaler but will be leveraged by the Gen3 scaler.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>