drm/amdgpu/gfx: switch to amdgpu_gfx_ras_late_init helper function

amdgpu_gfx_ras_late_init is used to init gfx specfic
ras debugfs/sysfs node and gfx specific interrupt handler.
It can be shared among gfx 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>