va: gstvaallocator close opened file descriptor

I think VAAPI closes properly the file descriptors, but VA not. I think it does not affect too much due to a surface buffer pool is used, so not many file descriptors are created actually, but I think this part is missing. Let me know what you think.

Can be checked with this pipeline:

GST_GL_API=gles2 GST_GL_PLATFORM=egl LIBVA_DRIVER_NAME=iHD gdb --args  gst-launch-1.0 -v filesrc location= <> ! matroskademux  ! "video/x-h264" ! vah264dec ! "video/x-raw(memory:DMABuf)" ! vapostproc ! "video/x-raw(memory:DMABuf),format=RGBA" ! glimagesink

This fixes #2470 (closed)