drm/radeon: fix vm deadlocks on cayman

Locking mutex in different orders just screams for
deadlocks, and some testing showed that it is actually
quite easy to trigger them.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>