drm/amdgpu: add reference for **fence

fix fence is released when pass to **fence sometimes.
add reference for it.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian K?nig <christian.koenig@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c b/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
index 6de7dc8..9de8104 100644
--- a/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
+++ b/drivers/gpu/drm/amd/amdgpu/sdma_v2_4.c
@@ -733,6 +733,7 @@
 	}
 
 err1:
+	fence_put(f);
 	amdgpu_ib_free(adev, &ib);
 err0:
 	amdgpu_wb_free(adev, index);