svga: implement surface cache size limit

svga: implement surface cache size limit

There was a SVGA_HOST_SURFACE_CACHE_BYTES symbol, but it was never
used.

Now when we go to add a newly deleted surface to the cache we check
if the cache size would be exceeded.  If so, try to free the least
recently "unused" surfaces until the cache is smaller.  If we can't
do that, simply don't cache the newly deleted surface.  The alternative
involves flushing and waiting and we don't want to do that.

Reviewed-by: José Fonseca <jfonseca@vmware.com>