dix: free default colormap before screen deletion

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>

dix/dispatch.c

@@ -3904,6 +3904,10 @@ RemoveGPUScreen(ScreenPtr pScreen)
     }
     screenInfo.numGPUScreens--;
 
+    /* this gets freed later in the resource list, but without
+     * the screen existing it causes crashes - so remove it here */
+    if (pScreen->defColormap)
+        FreeResource(pScreen->defColormap, RT_COLORMAP);
     free(pScreen);
 
 }