drm/amdgpu: getting fan speed pwm for vega10 properly

Instead of using RPM speed, we will use a function from vega20 based on PWM registers.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Yury Zhuravlev <stalkerg@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>