st/egl: reorganize backend initialization

st/egl: reorganize backend initialization

Remove set_event_handler() and pass the event handler with
native_get_XXX_platform().  Add init_screen() so that the pipe screen is
created later.  This way we don't need to pass user_data to
create_display().
(cherry picked from commit 73df31eedd0f33c8a9907855cb247c8f87964c48)