media: venus: flush all buffers in output plane streamoff

For scenarios, when source change is followed by VIDIOC_STREAMOFF
on output plane, driver should discard any queued OUTPUT
buffers, which are not decoded or dequeued.
Flush with HFI_FLUSH_INPUT does not have any actual impact.
So, fix it, by invoking HFI_FLUSH_ALL, which will flush all
queued buffers.

Fixes: 85872f86 ("media: venus: Mark last capture buffer")
Signed-off-by: Dikshita Agarwal <quic_dikshita@quicinc.com>
Tested-by: Nathan Hebert <nhebert@chromium.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>