[media] vivid: don't handle CEC_MSG_SET_STREAM_PATH

vivid shouldn't process the CEC_MSG_SET_STREAM_PATH message: this will confuse
userspace follower code because it isn't aware of the state change of becoming
an active source.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>