drm/amd/powerplay: refine phm_register_thermal_interrupt interface

currently, not all asics implement this callback function
so not return error to avoid powerplay initialize failed
in those asices

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>