media: uapi: Expose VP8 probability lengths as defines

These values will be used by various drivers implementing the VP8
stateless API.

This had been suggested by Ezequiel Garcia for the Cedrus VP8 driver.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>