channel-display-gst: Use h/w based decoders with Intel GPUs if possible

We first try to detect if an Intel GPU is available (by looking into
udev's database) and then probe Gstreamer's registry cache to see
if there is h/w based decoder (element) available for the incoming
video codec format. If both these conditions are satisfied (i.e,
Intel Media SDK Gstreamer plugin (libgstmsdk.so) and associated
libraries are properly installed), we then create a simple decode
pipeline using appropriate h/w based decoder and post-processor
elements instead of relying on playbin -- which may not be able to
auto-select these elements.

For example, if the incoming codec format is h264, we then create
a pipeline using msdkh264dec and vaapipostproc elements instead of
avdec_h264 and videoconvert.

Cc: Frediano Ziglio <freddy77@gmail.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Hazwan Arif Mazlan <hazwan.arif.mazlan@intel.com>
Signed-off-by: Jin Chung Teng <jin.chung.teng@intel.com>
Acked-by: Frediano Ziglio <freddy77@gmail.com>