media: mtk-vcodec: venc: support START and STOP commands

The V4L2 encoder specification requires encoders to support the
V4L2_ENC_CMD_START and V4L2_ENC_CMD_STOP commands. Add support for these
to the mtk-vcodec encoder by reusing the same flush buffer as used by
the decoder driver.

[hsinyi: fix double-free issue if flush buffer was not dequeued by the
time streamoff is called]

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>