drm/amdgpu: use GPU PAGE SHIFT for umc retired page

umc retired page belongs to vram and it should be aligned to gpu page
size

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>