drm/amdgpu/pm: update comment to clarify Overdrive interfaces

Vega10 and previous asics use one interface, vega20 and newer
use another.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>