drm/radeon: fix scratch reg handling for UVD fence

Also init the scratch reg to zero on the UVD ring.
This fixes UVD on AGP based cards.
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_fence.c

@@ -767,8 +767,8 @@ int radeon_fence_driver_start_ring(struct radeon_device *rdev, int ring)
 
 	radeon_scratch_free(rdev, rdev->fence_drv[ring].scratch_reg);
 	if (rdev->wb.use_event || !radeon_ring_supports_scratch_reg(rdev, &rdev->ring[ring])) {
-		if (ring != R600_RING_TYPE_UVD_INDEX) {
 		rdev->fence_drv[ring].scratch_reg = 0;
+		if (ring != R600_RING_TYPE_UVD_INDEX) {
 			index = R600_WB_EVENT_OFFSET + ring * 4;
 			rdev->fence_drv[ring].cpu_addr = &rdev->wb.wb[index/4];
 			rdev->fence_drv[ring].gpu_addr = rdev->wb.gpu_addr +