drm/amdgpu/powerplay: fix locking typo

Fixes: 2a507105 ("drm/amd/powerplay: add global PowerPlay mutex.")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>