drm/amdgpu: Program gds backup address as zero if no gds allocated

It is firmware requirement to set gds_backup_addrlo and gds_backup_addrhi
of DE meta both zero if no gds partition is allocated for the frame.

Signed-off-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>