vaapipostproc: Bob deinterlace doesn't work for raw file.

Submitted by wangfei @wangfei

Link to original bug (#796625)

Description

$gst-launch-1.0 -vf filesrc location=1920x1080.nv12 num-buffers=1 ! rawvideoparse format=nv12 width=1920 height=1080 ! vaapipostproc format=nv12 width=1920 height=1080 deinterlace-mode=1 deinterlace-method=bob ! checksumsink2 file-checksum=false frame-checksum=false plane-checksum=false dump-output=true dump-location=bob_NV12_1920x1080.yuv

The output yuv have 2 frames, and each frame is bit match with input.

But by using yamivpp:
$yamivpp --di bob 1920x1080.nv12 1920x1080.bob.nv12
the output only 1 frame, and md5 is different with input.