gstgldisplay: Close a small race starting the display thread
gstgldisplay: Close a small race starting the display thread

Take the display lock before signalling the create-context cond
to ensure the caller has dropped the lock and is therefore listening
for the signal.

mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl,ystreet/gst-plugins-gl,ystreet/gst-plugins-gl