drm: Add a hash-tab rcu-safe API

While hashtab should now be RCU-safe, Add a drm_ht_xxx_api for consumers
to use to make it obvious what locking mechanism is used.

Document the way the rcu-safe interface should be used.

Don't use rcu-safe list traversal in modify operations where we should use
a spinlock / mutex anyway.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>