drm/amd/powerplay: fix deadlock around smu_handle_task V2

As the lock was already held on the entrance to smu_handle_task.

- V2: lock in small granularity

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>