drm/amdgpu: Fix harvest reporting of VCN

Use VCN instance mask to check if an instance is harvested or not.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>