drm/radeon: fix vm deadlocks on cayman

commit bb4091558228ff4a3e02328c931e683fc7f08722 upstream.

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>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>