virgl: pass whole GL scanout dimensions

Spice client needs the whole GL texture dimension to be able to show a
scanout with a monitor offset (different than +0+0).

Furthermore, this fixes a crash when calling surface_{width,height}()
after dpy_gfx_replace_surface(con, NULL) was called in
virgl_cmd_set_scanout()

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1465911849-30423-4-git-send-email-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
