drm/powerplay: Fix Vega20 power reading again

For the 40.46 SMU release, they changed CurrSocketPower to
AverageSocketPower, but this was changed back in 40.47 so just check if
it's 40.46 and make the appropriate change

Tested with 40.45, 40.46 and 40.47 successfully

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>