drm/radeon: allow move_notify to be called without reservation

The few places that care should have those checks instead.
This allows destruction of bo backed memory without a reservation.
It's required for being able to rework the delayed destroy path,
as it is no longer guaranteed to hold a reservation before unlocking.

However any previous wait is still guaranteed to complete, and it's
one of the last things to be done before the buffer object is freed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>