drm/amdgpu: allocate shared fence slot in VA IOCTL

Per VM BOs share the reservation object with the PD and so need to
reserve a shared fence slot for the update.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>