media: venus: venc: Add support for constant quality control

Adds implementation of V4L2_CID_MPEG_VIDEO_CONSTANT_QUALITY v4l
control when the bitrate mode is CQ.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>