drm/amdgpu/jpeg1.0: fix idle work handler

On VCN 1.0, VCN and JPEG use the same worker thread so cancel
the vcn worker rather than jpeg.  On VCN 2.0 and newer
there are separate workers for each.

Fixes: 93df7487 ("drm/amdgpu/jpeg: cancel the jpeg worker")
Tested-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>