media: mediatek: vcodec: Report supported bitrate modes

The media driver supports constant bitrate mode only.
The supported rate control mode is reported through querymenu() and
s_ctrl() fails if non constant bitrate mode (e.g. VBR) is requested.

Signed-off-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>