Revert "Revert "drm/amdgpu: Fix NULL dereference in dpm sysfs handlers""

This reverts commit 2456c290. Got lost
during code rebase/merge. No need to port this to other branches.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>