media: hantro: Store VP9 bit depth in context

Now that we have proper infrastructure for postprocessing 10-bit
formats, store VP9 bit depth in context.

Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>