drm/amdgpu: check sdma ras funcs pointer before accessing

sdma ras funcs are not supported by ASIC prior
to vega20

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Le Ma <Le.Ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>