drm/amd/pp: Fix overflow when setup decf/pix/disp dpm table.

Clear the count in the single table setup function to
avoid missing any tables.

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>