dpy_get_ui_info() shouldn't be called if the underlying GPU doesn't
support it.

Before the assert() was added and the regression introduced, GTK code
used to get "zero" UI info, for ex with a simple VGA device. The assert
was added to prevent from calling when there are no console too. The
other display backend that calls dpy_get_ui_info() correctly checks that
pre-condition.

Calling dpy_set_ui_info() is "safe" in this case, it will simply return
an error that can be generally ignored.

Fixes: commit a92e7bb4 ("ui: add precondition for dpy_get_ui_info()")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>