media: videobuf2: always set buffer vb2 pointer

We need to always link allocated vb2_dc_buf back to vb2_buffer because
we dereference vb2 in prepare() and finish() callbacks.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>