vaapi: DISPLAY envvar as dependency

In a multiple video cards system, a X11 environment may have different VA
capabilities. This patch tracks the DISPLAY environment variable to
invalidates the GStreamer features cache. Also tracks WAYLAND_DISPLAY.

https://bugzilla.gnome.org/show_bug.cgi?id=770357

gst/vaapi/gstvaapi.c

@@ -57,7 +57,7 @@ static void
 plugin_add_dependencies (GstPlugin * plugin)
 {
   const gchar *envvars[] = { "GST_VAAPI_ALL_DRIVERS", "LIBVA_DRIVER_NAME",
-    NULL
+    "DISPLAY", "WAYLAND_DISPLAY", NULL
   };
   const gchar *kernel_paths[] = { "/dev/dri", NULL };
   const gchar *kernel_names[] = { "card", "render" };