ENGR00269619 mxc vout:improve vb handling for 3 field deinterlacing

ENGR00269619 mxc vout:improve vb handling for 3 field deinterlacing

We need 2 video buffers to get a deinterlaced frame in VDI low motion
mode or medium motion mode. When there is no enough video buffer in
the active list, no one triggers the video buffer timer, then users
may be blocked at dqueue buffer ioctrl if they are in blocking mode.
In order to fix this issue, we may peek the first available video
buffer in the queue list so that the buffer may be taken as a reference
video buffer to do deinterlacing. If there is no video buffer in the
queue list, we should make users be able to trigger the timer again
when they queue buffers to the driver.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
(cherry picked from commit 52d0e3f01afbf49d8d16225dede18cc71daa0570)