drm/amd/pm: correct hwmon power label name

only vangogh has 2 types of hwmon power node: "fastPPT" and "slowPPT",
the other asic only has 1 type of hwmon power node: "PPT".

Signed-off-by: Yang Wang <KevinYang.Wang@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>