media: MAINTAINERS: Amend venus Maintainers and Reviewers

Stan has stepped back from active venus development as a result I'd like to
volunteer my help in keeping venus maintained upstream.

Discussing with the qcom team on this we agreed

+M for Dikshita
+R for me

Many thanks to Stan for his hard work over the years from originating this
driver upstream to his many years of maintenance of it too.

Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Dikshita Agarwal <quic_dikshita@quicinc.com>
Acked-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Bryan O'Donoghue <bod@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>