media: venus: venc: configure entropy mode

H264 entropy mode can be selected via V4L2 API but is eventually not
applied. Configure encoder with selected mode, CALVC (def) or CABAC.

Note that hw/firmware also expects a CABAC model configuration which
currently doesn't have existing V4L2 API control. For now, use model_0
which seems always supported and so the default one.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>