drm/amdgpu: the warning dereferencing obj for nbio_v7_4

if ras_manager obj null, don't print NBIO err data

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>