drm/amd/powerplay: guard manual mode prerequisite for clock level force

Force clock level is for dpm manual mode only.

Reported-by: Candice Li <candice.li@amd.com>
Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jack Gui <Jack.Gui@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>