drm/amdgpu: Correct get_xcp_mem_id calculation

Current calculation only works for NPS4/QPX mode, correct it for
NPS4/CPX mode.

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