[media] coda: don't ever use subsampling ping-pong buffers as reconstructed reference buffers

On i.MX6, two subsampling ping-pong buffers are used for motion estimation and
deblocking They should not be counted as framebuffers, or they will be also used
to store reconstructed frames, causing visible artifacts in P-frames.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>