drm/amd/powerplay: fix deadlock issue for smu_force_performance_level

the smu->mutex is internal lock resource in sw-smu, some functions will use
it at the same time, so it maybe will cause deadlock issue.
this patch fix this issue in smu_force_performance_level function.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>