media: mediatek: vcodec: Handle VP9 superframe bitstream with 8 sub-frames

The VP9 bitstream uses superframes, which each contain 8 sub-frames,
enable accessing the last superframe by increasing the range of the index
vaidation as the maximum number of superframes is 8 and not 7, so that the
last sub-frame can be decoded normally with the stateful VP9 decoder.

Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>