media: rcar-vin: use scratch buffer and always run in continuous mode

Instead of switching capture mode depending on how many buffers are
available use a scratch buffer and always run in continuous mode. By
using a scratch buffer the responsiveness of the capture loop is
increased as it can keep running even if there are no buffers available
from userspace.

As soon as a userspace queues a buffer it is inserted into the capture
loop and returned as soon as it is filled. This is a improvement on the
previous logic where the whole capture loop was stopped and switched to
single capture mode if userspace did not feed the VIN driver buffers at
the same time it consumed them. To make matters worse it was difficult
for the driver to reenter continuous mode if it entered single mode even
if userspace started to queue buffers faster. This resulted in
suboptimal performance where if userspace where delayed for a short
period the ongoing capture would be slowed down and run in single mode
until the capturing process where restarted.

An additional effect of this change is that the capture logic can be
made much simple as we know that continuous mode will always be used.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>