media: platform: mtk-vcodec: Do not force /dev/videoX node number

Let's v4l2 framework use a free /dev/videoX node for decode and encoder.
For the decoder call video_register_device() before register the media
controller device so the mapping between ins correctly done.
Since the registering sequence has changed rework exiting errors case too.

Fixes: 590577a4 ("[media] vcodec: mediatek: Add Mediatek V4L2 Video Decoder Driver")
Fixes: 4e855a6e ("[media] vcodec: mediatek: Add Mediatek V4L2 Video Encoder Driver")

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>