vaapisink: fix initialization with "drm" display type.

vaapisink: fix initialization with "drm" display type.

Force early initializatin of the GstVaapiDisplay so that to make sure
that the sink element display object is presented first to upstream
elements, as it will be correctly featuring the requested display type
by the user.

Otherwise, we might end up in situations where a VA/X11 display is
initialized in vaapidecode, then we try VA/DRM display in vaapisink
(as requested by the "display" property), but this would cause a failure
because we cannot acquire a DRM display that was previously acquired
through another backend (e.g. VA/X11).