media: rcar-vin: Remove handling of user-space buffers when stopping

When returning buffers to user-space it's no longer needed to examine
the buffers given to hardware as recent changes guarantees the only
buffer present in the hardware registers when the driver is not in the
running state is the scratch buffer.

Remove the special case and rename the function to better describe it
now only deals with buffers queued to the driver but not yet recorded in
the hardware registers.

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>