vaapipostproc: Enable to use DMABuf mem at sink and src pad
Add 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, gstreamer-vaapi!393