media: rkvdec: h264: Fix dpb_valid implementation

The ref builder only provided references that are marked as valid in the
dpb. Thus the current implementation of dpb_valid would always set the
flag to 1. This is not representing missing frames (this is called
'non-existing' pictures in the spec). In some context, these non-existing
pictures still need to occupy a slot in the reference list according to
the spec.

Fixes: cd33c830 ("media: rkvdec: Add the rkvdec driver")
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>