drm/amdgpu: Fix VCN allocation in CPX partition

VCN need not be shared in CPX mode always for all GFX 9.4.3 SOC SKUs. In
certain configs, VCN instance can be exclusively allocated to a
partition even under CPX mode.

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