media: hantro: Store HEVC bit depth in context

Store HEVC bit depth in context.
Bit depth is equal to hevc sps bit_depth_luma_minus8 + 8.

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