media: hantro: Remove unneeded hantro_dec_buf_finish

Since now .buf_prepare takes care of setting the
buffer payload size, we can get rid of this,
at least for decoders.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>