drm/amdgpu: move get_xgmi_relative_phy_addr to amdgpu_xgmi.c

centralize all the xgmi related function to amdgpu_xgmi.c

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>