drm/amdgpu/powerplay: enable sysfs and debugfs interfaces late

To avoid users accessing them before the module has finished
initializing them and make sure they are only created if
dpm has properly initialized.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>