media: mtk-vcodec: remove unneeded proxy functions

We were getting the codec interface through a proxy function that does
not bring anything compared to just accessing the interface definition
directly, so just do that. Also make the decoder interfaces const.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>