media: rkvdec: Enable capture buffer holding for H264

In order to support interlaced video decoding, the driver must
allow holding the capture buffer so that the second field can
be decoded into it.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>