drm/amd/pm: remove unused message SMU_MSG_SpareX

the SpareX is reserved by SMU firmware, the driver is never use it.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>