drm/amdgpu: avoid ras error injection for retired page

check whether a page is bad page before umc error injection, bad page
should not be accessed again

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>