drm/amdgpu: increase RAS bad page threshold

For default policy, driver will issue an RMA event when the number of
bad pages is greater than 8 physical rows, rather than reaches 8
physical rows, don't rely on threshold configurable parameters in
default mode.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>