drm/amdgpu/swSMU: handle manual AC/DC notifications

For boards that do not support automatic AC/DC transitions
in firmware, manually tell the firmware when the status
changes.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/1043


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