egl: Use a boolean to indicate whether a display is initialized.

The driver pointer of the display was used to decide whether a display
is initialized.  Use a boolean for that purpose allows accessing the
driver of an uninitialized display.

src/egl/main/egldisplay.h

@@ -49,6 +49,7 @@ struct _egl_display
 
    EGLNativeDisplayType NativeDisplay;
 
+   EGLBoolean Initialized; /**< True if the display is initialized */
    _EGLDriver *Driver;
    void *DriverData; /* private to driver */