media: uapi: h264: Clean DPB entry interface

As discussed recently, the current interface for the
Decoded Picture Buffer is not enough to properly
support field coding.

This commit introduces enough semantics to support
frame and field coding, and to signal how DPB entries
are "used for reference".

Reserved fields will be added by a follow-up commit.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>