media: mtk-vcodec: use v4l2_m2m_buffer where appropriate

Despite using M2M in both the decoder and encoder, this driver used
vb2_v4l2_buffer as its base buffer structure, and placed a list_head
right after the buffer declaration in order to match the layout of a
v4l2_m2m_buffer. This is very dangerous as it means the driver will
break should the layout of v4l2_m2m_buffer change.

Fix this by directly using v4l2_m2m_buffer and updating the sites that
accessed the buffer accordingly.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix checkpatch warning]
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>