drm/amdgpu: Use SPX as default in partition config

In certain cases - ex: when a reset is required on initialization - XCP
manager won't have a valid partition mode. In such cases, use SPX as the
default selected mode for which partition configuration details are
populated.

Fixes: 4ae86dc8 ("drm/amdgpu: Add sysfs nodes to get xcp details")
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reported-by: Hao Zhou <hao.zhou@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>