egl-helpers: add missing error check

Code didn't check for qemu_egl_init_dpy_mesa() failures, add it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170505104101.30589-5-kraxel@redhat.com
