media: rcar-vin: Break out hardware start and stop to new methods

To support suspend and resume the ability to start and stop the hardware
needs to be available internally in the driver. Currently this code is
in the start and stop callbacks of the vb2_ops struct. In these
callbacks the code is intertwined with buffer allocation and freeing.

Prepare for suspend and resume support by breaking out the hardware
start/stop code into new methods.

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