drm/amd/powerplay: avoid deadlock on Vega20 swSMU routine

The lock required was already hold by its parent API.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>