drm/amdgpu: Move calculation of xcp per memory node

Its value is required for finding the memory id of xcp.

Fixes: d26ea1b3 ("drm/amdgpu: Add xcp manager num_xcp_per_mem_partition")
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>