Fix uvm_object reference counting. While these reference counts aren't reaaly

Fix uvm_object reference counting.  While these reference counts aren't reaaly
used (ttm bo's have their own reference counts), we can't let the reference
count go negative as this will freak out the upper uvm layers.  Since the
uvm_object reference count is still a useful debugging tool (ddb will display
it for example), adjust it such that the uvm_object reference count represents
the number of references held by the uvm layer.

tested by matthieu@
ok jsg@