drm/vmwgfx: Adapt validation code for reference-free lookups

Adapt the validation code so that vmw_validation_add[res|bo] can be called
under an rcu read lock (non-sleeping) and with rcu-only protected resource-
or buffer object pointers.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>