gstreamer1.0-vaapi: downgrade vaapisink to marginal rank

gstreamer1.0-vaapi: downgrade vaapisink to marginal rank

Using vaapisink (which doesn't supports DRI3 [1] and uses DRI2) with
default poky configuration currently results in an unresponsive display
because DRI2 rendering doesn't work (as of xserver 1.20.3) in non-composited
environments [2].

Downgrade vaapisink to marginal for now so playbin (and in turn gst-play
and gtk-play examples) uses next best sink element and works out of box.

[1] https://github.com/intel/libva/issues/122
[2] https://gitlab.freedesktop.org/xorg/xserver/issues/13

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>