Check if there really is a valid offset for the BO.

v2: user WARN_ON_ONCE
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>