drm/amd/pm: fix uninitialized variable warning for smu8_hwmgr

Clear warnings that using uninitialized value level when fails
to get the value from SMU.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>