drm/amdgpu: Only create err_count sysfs when hw_op is supported

Some IP blocks only support partial ras feature and don't
have ras counter and/or ras error status register at all.
Driver should not create err_count sysfs node for those
IP blocks.

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