media: mtk-vcodec: Remove extra area allocation in an input buffer on encoding

MediaTek encoder allocates non pixel data area for an input buffer every
plane. As the input buffer should be read-only, the driver should not write
anything in the buffer. Therefore, the extra data should be unnecessary.

Signed-off-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>