media: mtk-vcodec: fix access to incorrect planes member

Commit 0650a914 ("media: mtk-vcodec: Correct return type for mem2mem
buffer helpers") fixed the return types for mem2mem buffer helper
functions by changing a few local variables from vb2_buffer to
vb2_v4l2_buffer. However, it left a few accesses to vb2_buffer::planes
as-is, accidentally turning them into accesses to
vb2_v4l2_buffer::planes and resulting in values being read from/written
to the wrong place.

Fix this by inserting vb2_buf into these accesses so they mimic their
original behavior.

Fixes: 0650a914 ("media: mtk-vcodec: Correct return type for mem2mem buffer helpers")

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>