drm/amdgpu: Reserve space for shared fence

Call reservation_object_reserve_shared to reserve
space for shared fence. Otherwise it will trigger
BUG_ON condition in reservation_object_add_shared_fence.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>