drm/amd/powerplay: fix bug dpm can't work when resume back on Polaris

Need to initialize the values to 0 since they get or'ed with additional
values.  If the initialization is missing, on resume, they may end up
with a combination of stale data and new data.

Fixes dpm on resume.

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