drm/amdgpu: fix locking in force performance level

Looks like a copy paste typo when we added powerplay
support.

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>