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.