drm/amd/powerplay: fix mclk can't switch on Tonga

regression issue caused by
commit 47047263
("drm/amd/powerplay: delete eventmgr related files.")

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