drm/ttm: Optimize vm locking using kref_get_unless_zero v3

Removes the need for a write lock each time we call ttm_bo_unref().

v2: Remove an unused variable.
v3: Really remove the unused variable.

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