vaapipostproc: Enable to use DMABuf mem at sink and src pad

vaapipostproc: Enable to use DMABuf mem at sink and src pad

Enable DMABuf mem at both sink and src pad for vaapipostproc caps which
helps to apply DMABuf for more generic use cases: such as "! vaapih264dec !
video/x-raw(memory:DMABuf) ! vaapipostproc ! video/x-raw (memory:DMABuf)
! vaapih265enc"  . Or other mem converions like from DMABuf to VASurface
through vaapipostproc.

This patch is a portion from Junyan.He@intel.com
https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/393

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1287>