drm/amdgpu: Allow kfd CRIU with no buffer objects

The kfd CRIU checkpoint ioctl would return an error if trying
to checkpoint a process with no kfd buffer objects.

This is a normal case and should not be an error.

Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>