FROMLIST: media: mtk-vcodec: vdec: add media device if using stateless api

The stateless API requires a media device for issuing requests. Add one
if we are being instantiated as a stateless decoder.

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
[acourbot: refactor, cleanup and split]
Co-developed-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Alexandre Courbot <acourbot@chromium.org>