drm/amdgpu: fix using shared fence for exported BOs v2

It is perfectly possible that the BO list is created before the BO is
exported. While at it clean up setting shared to one instead of true.

v2: add comment and simplify logic

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>