GLX: Remove evdpy / dpy split

Move to one display. There's no reason to have two displays here -- the
comment stated that one should touch GLX and one should touch window
events, and that they should be touched from different threads, but the
current code wasn't this careful.

Just use one Display connection.

Source/Core/DolphinWX/GLInterface/GLInterface.h

@@ -33,9 +33,7 @@ typedef struct {
 	int screen;
 	Window win;
 	Window parent;
-	// dpy used for glx stuff, evdpy for window events etc.
-	// evdpy is to be used by XEventThread only
-	Display *dpy, *evdpy;
+	Display *dpy;
 	XVisualInfo *vi;
 	XSetWindowAttributes attr;
 	std::thread xEventThread;