drm/amdgpu: Fix XCP instance mask calculation

Fix instance mask calculation for VCN IP. There are cases where VCN
instance could be shared across partitions. Fix here so that other
blocks don't need to check for any shared instances based on partition
mode.

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