drm/amd/powerplay/vega20: enable fan RPM and pwm settings V2

Manual fan RPM and pwm setting on vega20 are
available now.

V2: correct the register for fan speed setting and
    avoid divide-by-zero

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