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)
Status Name Job ID Coverage
  Preparation
passed gst indent #79565

00:00:19

passed manifest #79564

00:00:29

 
  Build
passed build fedora x86_64 #79566

00:03:40

 
  Test
passed check fedora #79567

00:03:38

 
  Full Builds
passed build android arm64 api28 #79568

00:05:23

passed build cerbero cross win32 #79570

00:07:28

passed build cerbero cross win64 #79571

00:08:41

passed build cerbero fedora x86_64 #79569

00:07:59