drm/amd/pm: correct the fan speed RPM retrieving

The relationship "PWM = RPM / smu->fan_max_rpm" between fan speed
PWM and RPM is not true for SMU11 ASICs. So, we need a new way to
retrieving the fan speed RPM.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>