venus: venc: add handling for VIDIOC_ENCODER_CMD

Add handling for below commands in encoder:
1. V4L2_ENC_CMD_STOP
2. V4L2_ENC_CMD_START

Signed-off-by: Dikshita Agarwal <quic_dikshita@quicinc.com>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>