drm/radeon/dpm: Fix hwmon crash

Commit ec39f64b (drm/radeon/dpm: Convert
to use devm_hwmon_register_with_groups) converted one usage of
dev_get_drvdata, but there were two more.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=72457



Signed-off-by: Martin Andersson <g02maran@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>