plugins: postproc: Fix a problem of propose_allocation when passthrough.
We should query the downstream element to answer a precise allocation
query when the passthrough mode is enabled.
The current way still decides the allocation by the postproc itself. The
pipeline such as:
gst-launch-1.0 -v filesrc location=xxx.264 ! h264parse ! vaapih264dec !
vaapipostproc ! fakevideosink silent=false sync=true
will lose some info such as the GST_VIDEO_META_API_TYPE.