vaapipostproc: before set surface proxy, check if it already been created and exist.

Fix the deinterlace black frame when playing with glimagesink:
gst-launch-1.0 filesrc location=test.264 ! h264parse ! vaapih264dec \
! vaapipostproc deinterlace-mode=1 deinterlace-method=1 ! glimagesink
8 jobs for deint in 16 minutes and 33 seconds (queued for 3 seconds)