media: staging: tegra-vde: Properly mark invalid entries

Entries in the reference picture list are marked as invalid by setting
the frame ID to 0x3f.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>