drm/amdgpu: Use mask for active clusters

Use a mask of available active clusters instead of using only the number
of active clusters.

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