media: v4l2-ctrls: Add encoder constant quality control

When V4L2_CID_MPEG_VIDEO_BITRATE_MODE value is
V4L2_MPEG_VIDEO_BITRATE_MODE_CQ, encoder will produce
constant quality output indicated by
V4L2_CID_MPEG_VIDEO_CONSTANT_QUALITY control value.
Encoder will choose appropriate quantization parameter
and bitrate to produce requested frame quality level.

Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>