media: mtk-vcodec: make flush buffer reusable by encoder

The flush buffer is a special buffer that tells the decoder driver to
send an empty CAPTURE frame to the client with V4L2_BUF_FLAG_LAST set.

We need similar functionality for the encoder ; however currently the
flush buffer depends on decoder-specific structures and thus cannot be
reused with the encoder.

Fix this by testing for this buffer by its VB2 address, and not through
a dedicated flag stored in a higher-level decoder structure. This also
allows us to remove said flag and simplify the code a bit.

Since the flush buffer should never be used in the stateless decoder,
also add safeguards to check against it.

Signed-off-by: Alexandre Courbot <acourbot@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>