drm/amdgpu/swsmu: use percent rather than rpm for manual fan control

On some boards the rpm interface apparently does not work at all
leading to the fan not spinning or spinning at strange speeds.
Both interfaces work properly on the boards I have.
Let's try and use the percent interface instead.

Bug: drm/amd#1408


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