hantro: Rework how encoder and decoder are identified

So far we've been using the .buf_finish hook to distinguish
decoder from encoder. This is unnecessarily obfuscated.

Moreover, we want to move the buf_finish, so use a cleaner
scheme to distinguish the driver decoder/encoder type.

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