drm/amdgpu/mmhub: switch to amdgpu_mmhub_ras_late_init helper function

amdgpu_mmhub_ras_late_init is used to init mmhub specfic
ras debugfs/sysfs node and mmhub specific interrupt handler.
It can be shared among mmhub generations

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>