vaapisink: add support for "display-name" property.

vaapisink: add support for "display-name" property.

Add a "display-name" property to vaapisink so that the end user could
select the desired output. Keep "display-name" in-line with the existing
"display" (GstVaapiDisplayXXX type).

So, for X11 or GLX, the "display-name" is the usual display name as we
know for XOpenDisplay(); for Wayland, the "display-name" is the name used
for wl_display_connect(); and for DRM, the "display-name" is actually the
DRI device name.

https://bugzilla.gnome.org/show_bug.cgi?id=722247