drm/amd/powerplay: Zero out power data in AMDGPU_PP_SENSOR_GPU_POWER readout

Since 12 of the 16 bytes are not initialized with anything let's ensure they're
sensibly zeroed out otherwise debugfs callers will read back garbage
(because they assume debugfs wrote sensible data back...)

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>