[intel-gem] Encourage multiple caches to hold read data

When reading from multiple domains, allow each cache to continue
to hold data until writes occur somewhere. This is done by
first leaving the read_domains alone at bind time (presumably the CPU read
cache contains valid data still) and then in set_domain, if no write_domain
is specified, the new read domains are simply merged into the existing read
domains.

A huge comment was added above set_domain to explain how things are
expected to work.