drm/amdgpu/powerplay: implement fan control interface in amd_powerplay_funcs

This adds the interface needed to expose powerplay fan control to sysfs
via hwmon.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>