This fixes the regression introduced in 64acc74.
If a pad supports multiple set of capsfeatures, it needs to add
multiple equal structures with different feature sets to the caps.
Because caps structures with the same name but with a non-equal
set of caps features are not compatible.
Without this patch, playbin will autoplug xvimagesink instead of vaapisink.
https://bugzilla.gnome.org/show_bug.cgi?id=750095
