drm/amdgpu: fix error handling in cz_dpm_hw_fini/cz_dpm_suspend

Need to unlock the mutex on error.

Noticed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>