drm/amd/pm: do not use drm middle layer for debugfs

Use debugfs API directly instead of drm middle layer.

v2: * checkpatch.pl: use '0444' instead of S_IRUGO.
    * remove S_IFREG from mode.
    * remove mode variable.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>