media: hantro: h264: Make dpb entry management more robust

The driver maintains stable slot locations for reference pictures. This
change makes the code more robust by using the reference_ts as key and
by marking all entries invalid right from the start.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>