x11: Move the EGL display into the private struct

x11: Move the EGL display into the private struct

There's no need to use g_object_set_data() for it.

We can also stop caching it elsewhere because we know the display has
it.

And finally, we can remove the display->have_egl boolean and use
display->egl_display != NULL instead. We initialize the display at
startup, so that variable is the perfect indicator.