drm/amdgpu: Fix false positive error log

It should first check block ras obj whether be set, it should
return 0 directly if block ras obj or hw_ops is not set.
If block doesn't support RAS just return 0 is fine.

Changed from V1:
	return 0 directly if block ras obj or hw ops is not set

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