[media] rcar-vin: select capture mode based on free buffers

Instead of selecting single or continuous capture mode based on how many
buffers userspace intends to give us select capture mode based on number
of free buffers we can allocate to hardware when the stream is started.

This change is a prerequisite to enable the driver to switch from
continuous to single capture mode (or the other way around) when the
driver is stalled by userspace not feeding it buffers as fast as it
consumes it.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>