drm/amd/powerplay: fix pcie sysfs interface when set wrong value

The operation of mutex_unlock smu->mutex should be done when forced
level is larger than NUM_LINK_LEVELS in the function of force_clk_levels.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>