drm/radeon/kms: IB locking dumps out a lockdep ordering issue

We sometimes lock IB then the ring and sometimes the ring then
the IB. This is mostly due to the IB locking not being well defined
about what data in the structs it actually locks. Define what I
believe is the correct behaviour and gets rid of the lock dep ordering
warning.

Signed-off-by: Dave Airlie <airlied@redhat.com>