media: mtk-vcodec: venc support MIN_OUTPUT_BUFFERS control

This control is required by v4l2-compliance for encoders. A value of 1
should be suitable for all scenarios.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>