drm/amd/pm: drop unneeded lock protection smu->mutex

As all those APIs are already protected either by adev->pm.mutex
or smu->message_lock.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>