drm/amdgpu: Add FRU sysfs nodes only if needed

Create sysfs nodes for FRU data only if FRU data is available. Move the
logic to FRU specific file.

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>