media: staging: tegra-vde: Align bitstream size to 16K

I've noticed that decoding fails sometime if size of bitstream buffer
isn't aligned to 16K, probably because HW fetches data from memory in
a 16K granularity and if the last chunk of data isn't aligned, HW reads
garbage data beyond the dmabuf and tries to parse it.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>