drm/amdgpu: add warning to amdgpu_bo_gpu_offset() v2

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

v2: user WARN_ON_ONCE

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>