media: hantro: Empty encoder capture buffers by default

The payload size for encoder capture buffers is set by the driver upon
finishing encoding each frame, based on the encoded length returned from
hardware, and whatever header and padding length used. Setting a
non-zero default serves no real purpose, and also causes issues if the
capture buffer is returned to userspace unused, confusing the
application.

Instead, always set the payload size to 0 for encoder capture buffers
when preparing them.

Fixes: 775fec69 ("media: add Rockchip VPU JPEG encoder driver")
Fixes: 082aaecf ("media: hantro: Fix .buf_prepare")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>