console: add dpy_gl_scanout_disable

Helper function (and DisplayChangeListenerOps ptr) to disable scanouts.
Replaces using dpy_gl_scanout_texture with 0x0 size and no texture
specified.

Allows cleanups to make the io and gfx emulation code more readable.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1487669841-13668-3-git-send-email-kraxel@redhat.com