drm/amdgpu: Initialize xcc mask

For ASICs which are not initialized through discovery, initialize GFX
cluster as 1.

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>