media: mtk-vcodec: vdec: use helpers in VIDIOC_(TRY_)DECODER_CMD

Let's use the dedicated helpers to make sure we get the expected
behavior and remove redundant code.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>