drm/amdgpu: add RAS fatal error interrupt handler

The fatal error handler is independent from general ras interrupt
handler since there is no related IH ring.

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>