drm/amdgpu: Remove duplicated power source update

when ac/dc switch, driver will be notified by acpi event.
then the power source will be updated. so don't need to
get power source when set power state.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>