drm/amdgpu: export fan control functions to amdgpu

Hook up the amdgpu thermal control callbacks for powerplay.

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>