drm/radeon: unlock the ring mutex while waiting for the next fence

Fixing just another deadlock problem with gpu reset tests.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon_ring.c b/drivers/gpu/drm/radeon/radeon_ring.c
index 08e1578..407d90a 100644
--- a/drivers/gpu/drm/radeon/radeon_ring.c
+++ b/drivers/gpu/drm/radeon/radeon_ring.c
@@ -346,7 +346,9 @@
 		if (ndw < ring->ring_free_dw) {
 			break;
 		}
+		mutex_unlock(&ring->mutex);
 		r = radeon_fence_wait_next(rdev, radeon_ring_index(rdev, ring));
+		mutex_lock(&ring->mutex);
 		if (r)
 			return r;
 	}