If we don't free this here, it gets freed later in the resource
cleanups, however it then looks up up pmap->pScreen, which we
freed already in this function. So free the default colormap
when we should.
This fixes a bug after a couple of hotplug cycles when you try
to exit the X server and it crashes.
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>