drm/amdgpu: decouple sysfs creating of bad page node

Bad page information should not be exposed by sysfs when
bad page retirement is disabled, so decouple it from ras
sysfs group creating, and add one guard before creating.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>