drm/amd/powerplay: use hardware fan control if no powerplay fan table

Otherwise, you may get divided-by-zero error or corrput the SMU fan
control feature.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Slava Abramov <slava.abramov@amd.com>
Acked-by: Slava Abramov <slava.abramov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>