egl/dri2: Log a warning if no platforms are enabled.

If you hit this, you didn't compile with --with-egl-platforms=...

Recompile with something like --with-egl-platforms=x11,drm and make
clean and make again.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/egl/drivers/dri2/egl_dri2.c

@@ -659,6 +659,7 @@ dri2_initialize(_EGLDriver *drv, _EGLDisplay *disp)
 #endif
 
    default:
+      _eglLog(_EGL_WARNING, "No EGL platform enabled.");
       return EGL_FALSE;
    }
 }