drm/amdgpu: Fix Arcturus fan speed reporting

Arcturus doesn't have a fan. The assumption of "if the manual fan
control bit isn't set, it's on automatic mode" does not hold true if the
fan is missing, and results in exposing an invalid value for fan speed.

The SMU metrics table accurately reflects the lack of fan and will
return 0 for the fan speed. Trying to use the
smu_v11_0_get_fan_speed_rpm function will return invalid data, so just
stick with the SMU metrics for Arcturus

Signed-off-by: Kent Russell <kent.russell@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>