drm/radeon: do not use drm middle layer for debugfs (v2)

Use debugfs API directly instead of drm middle layer.

v2: squash in build fix (Alex)

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