drm/amd/pm: fix the deadlock observed on performance_level setting

The sub-routine(amdgpu_gfx_off_ctrl) tried to obtain the lock
adev->pm.mutex which was actually hold by amdgpu_dpm_force_performance_level.
A deadlock happened then.

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