media: venus: venc: drop VP9 codec support

No one of the supported Venus version has implemented VP9 codec
for enconding, so drop it from the list of codecs.

Signed-off-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>